In CoreAdminHome on the generalSettings page there is a link to "how to setup cron". The template has a small error: https://github.com/piwik/piwik/blob/master/plugins/CoreAdminHome/templates/generalSettings.tpl#L24
There is an additional "php?" in front of url= which makes the Proxy not redirect properly (as the GET-param "url" is not set).
(In ) Fixes #1946