wrong configuration for discovery service: 'idp' must be 'idpentity' in config/authsources.php

[iv-enovation]

in config/authsources.php in line 65:
'idp' => empty($CFG->auth_saml2_disco_url) ? $idpentityid : null,

is wrong, it must be:
'idpentityid' => empty($CFG->auth_saml2_disco_url) ? $idpentityid : null,

as further in .extlib/simplesamlphp/modules/saml/www/sp/discoresp.php it checks for 'idpentity':
if (!array_key_exists('idpentityid', $_REQUEST)) {
throw new \SimpleSAML\Error\BadRequest('Missing idpentityid to discovery service response handler');
}