Hey together,
we solved this issue thanks to the debugging of one of our team members.
The issue was caused in our version of the index.php of the kirbyTextRaw-plugin where we had a new line on line 1:
<?php
/*
* Kirby 3 plugin - kirbytextRaw
* Parse Markdown-formatted text as kirbytext without enclosing <p> tags
*
This caused headers to be sent and the particular php.ini of the hoster didn’t like it.
this must be a classic. So beware, dear readers!
Thanks for all the support and this great piece of software!