Invalid header line detected

[falco-knapp]

A customer reported the following issue:

During loading of the attribute-mapping of field Inxmail Professional recipient the generic error message "Invalid header line detected" appeared.

There is no evidence, that the issue is related with Inxmail Professional. The error message seems to be quite related to Magento:
https://www.google.com/search?q=Invalid+header+line+detected

Stack-Trace:
#0 /Users/user/Workspace/magento2/customer/project/src/vendor/inxmail/magento2-module/Model/Api/ApiClient.php(350): Zend_Http_Response::extractHeaders('HTTP/2 200 \r\nda...') #1 /Users/user/Workspace/magento2/customer/project/src/vendor/inxmail/magento2-module/Model/Request/RequestRecipientAttributes.php(69): Flagbit\Inxmail\Model\Api\ApiClient->getResource('', '', NULL, NULL) #2 /Users/user/Workspace/magento2/customer/project/src/vendor/inxmail/magento2-module/Block/Adminhtml/System/Config/AttribSelectInx.php(64): Flagbit\Inxmail\Model\Request\RequestRecipientAttributes->sendRequest() #3 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/AbstractBlock.php(667): Flagbit\Inxmail\Block\Adminhtml\System\Config\AttribSelectInx->_toHtml() #4 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Field/FieldArray/AbstractFieldArray.php(213): Magento\Framework\View\Element\AbstractBlock->toHtml() #5 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/view/adminhtml/templates/system/config/form/field/array.phtml(54): Magento\Config\Block\System\Config\Form\Field\FieldArray\AbstractFieldArray->renderCellTemplate('inxAttrib') #6 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/TemplateEngine/Php.php(59): include('/Users/user...') #7 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/Template.php(270): Magento\Framework\View\TemplateEngine\Php->render(Object(Flagbit\Inxmail\Block\Adminhtml\System\Config\AttributeMapping), '/Users/user...', Array) #8 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/Template.php(300): Magento\Framework\View\Element\Template->fetchView('/Users/user...') #9 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-backend/Block/Template.php(126): Magento\Framework\View\Element\Template->_toHtml() #10 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Field/FieldArray/AbstractFieldArray.php(262): Magento\Backend\Block\Template->_toHtml() #11 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Field/FieldArray/AbstractFieldArray.php(116): Magento\Config\Block\System\Config\Form\Field\FieldArray\AbstractFieldArray->_toHtml() #12 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Field.php(78): Magento\Config\Block\System\Config\Form\Field\FieldArray\AbstractFieldArray->_getElementHtml(Object(Magento\Framework\Data\Form\Element\Text)) #13 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Field.php(52): Magento\Config\Block\System\Config\Form\Field->_renderValue(Object(Magento\Framework\Data\Form\Element\Text)) #14 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form/Element/AbstractElement.php(459): Magento\Config\Block\System\Config\Form\Field->render(Object(Magento\Framework\Data\Form\Element\Text)) #15 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form/Element/Text.php(41): Magento\Framework\Data\Form\Element\AbstractElement->getHtml() #16 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form/Element/AbstractElement.php(473): Magento\Framework\Data\Form\Element\Text->getHtml() #17 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Fieldset.php(86): Magento\Framework\Data\Form\Element\AbstractElement->toHtml() #18 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/Block/System/Config/Form/Fieldset.php(66): Magento\Config\Block\System\Config\Form\Fieldset->_getChildrenElementsHtml(Object(Magento\Framework\Data\Form\Element\Fieldset)) #19 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form/Element/AbstractElement.php(459): Magento\Config\Block\System\Config\Form\Fieldset->render(Object(Magento\Framework\Data\Form\Element\Fieldset)) #20 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form/Element/AbstractElement.php(473): Magento\Framework\Data\Form\Element\AbstractElement->getHtml() #21 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form.php(325): Magento\Framework\Data\Form\Element\AbstractElement->toHtml() #22 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/Data/Form.php(340): Magento\Framework\Data\Form->toHtml() #23 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-backend/Block/Widget/Form.php(101): Magento\Framework\Data\Form->getHtml() #24 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-backend/view/adminhtml/templates/widget/form.phtml(15): Magento\Backend\Block\Widget\Form->getFormHtml() #25 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/TemplateEngine/Php.php(59): include('/Users/user...') #26 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/Template.php(270): Magento\Framework\View\TemplateEngine\Php->render(Object(Magento\Config\Block\System\Config\Form), '/Users/user...', Array) #27 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/Template.php(300): Magento\Framework\View\Element\Template->fetchView('/Users/user...') #28 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-backend/Block/Template.php(126): Magento\Framework\View\Element\Template->_toHtml() #29 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/AbstractBlock.php(667): Magento\Backend\Block\Template->_toHtml() #30 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(557): Magento\Framework\View\Element\AbstractBlock->toHtml() #31 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(533): Magento\Framework\View\Layout->_renderBlock('system\\config\\f...') #32 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('system\\config\\f...') #33 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('system\\config\\f...') #34 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('system\\config\\f...', true) #35 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/AbstractBlock.php(507): Magento\Framework\View\Layout\Interceptor->renderElement('system\\config\\f...', true) #36 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-config/view/adminhtml/templates/system/config/edit.phtml(22): Magento\Framework\View\Element\AbstractBlock->getChildHtml('form') #37 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/TemplateEngine/Php.php(59): include('/Users/user...') #38 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/Template.php(270): Magento\Framework\View\TemplateEngine\Php->render(Object(Magento\Config\Block\System\Config\Edit), '/Users/user...', Array) #39 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/Template.php(300): Magento\Framework\View\Element\Template->fetchView('/Users/user...') #40 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/module-backend/Block/Template.php(126): Magento\Framework\View\Element\Template->_toHtml() #41 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Element/AbstractBlock.php(667): Magento\Backend\Block\Template->_toHtml() #42 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(557): Magento\Framework\View\Element\AbstractBlock->toHtml() #43 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(533): Magento\Framework\View\Layout->_renderBlock('system.config.e...') #44 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('system.config.e...') #45 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('system.config.e...') #46 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('system.config.e...', false) #47 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('system.config.e...', false) #48 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('content', false) #49 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('content') #50 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('content') #51 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('content', false) #52 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('content', false) #53 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('admin.scope.col...', false) #54 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('admin.scope.col...') #55 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('admin.scope.col...') #56 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('admin.scope.col...', false) #57 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('admin.scope.col...', false) #58 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('main.col', false) #59 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('main.col') #60 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('main.col') #61 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('main.col', false) #62 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('main.col', false) #63 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('page.main.conta...', false) #64 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('page.main.conta...') #65 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('page.main.conta...') #66 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('page.main.conta...', false) #67 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('page.main.conta...', false) #68 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('page.content', false) #69 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('page.content') #70 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('page.content') #71 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('page.content', false) #72 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('page.content', false) #73 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('page.wrapper', false) #74 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('page.wrapper') #75 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('page.wrapper') #76 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('page.wrapper', false) #77 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('page.wrapper', false) #78 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('backend.page', false) #79 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('backend.page') #80 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('backend.page') #81 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('backend.page', false) #82 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(585): Magento\Framework\View\Layout\Interceptor->renderElement('backend.page', false) #83 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(535): Magento\Framework\View\Layout->_renderContainer('root', false) #84 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(206): Magento\Framework\View\Layout->renderNonCachedElement('root') #85 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(488): Magento\Framework\View\Layout\Interceptor->renderNonCachedElement('root') #86 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(193): Magento\Framework\View\Layout->renderElement('root', true) #87 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Layout.php(954): Magento\Framework\View\Layout\Interceptor->renderElement('root') #88 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/View/Layout/Interceptor.php(492): Magento\Framework\View\Layout->getOutput() #89 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Result/Page.php(257): Magento\Framework\View\Layout\Interceptor->getOutput() #90 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/View/Result/Layout.php(170): Magento\Framework\View\Result\Page->render(Object(Magento\Framework\App\Response\Http\Interceptor)) #91 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Backend/Model/View/Result/Page/Interceptor.php(193): Magento\Framework\View\Result\Layout->renderResult(Object(Magento\Framework\App\Response\Http\Interceptor)) #92 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/App/Http.php(139): Magento\Backend\Model\View\Result\Page\Interceptor->renderResult(Object(Magento\Framework\App\Response\Http\Interceptor)) #93 /Users/user/Workspace/magento2/customer/project/src/generated/code/Magento/Framework/App/Http/Interceptor.php(24): Magento\Framework\App\Http->launch() #94 /Users/user/Workspace/magento2/customer/project/src/vendor/magento/framework/App/Bootstrap.php(257): Magento\Framework\App\Http\Interceptor->launch() #95 /Users/user/Workspace/magento2/customer/project/src/pub/index.php(37): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http\Interceptor)) #96 /Users/userffp/.composer/vendor/thirdparty/valet-plus/server.php(131): require('/Users/user...') #97 {main}

[falco-knapp]

Additional Information:
Issue exists apparently with:
PHP Version 7.0
PHP Version 7.1
Magento Version 2.2.7.