Installation on Ionos server

I am trying to install kirby on an Ionos Server.
I have changed the .htaccess file and updated the permissions for folders and files.
The php version is 8.2.
Result: I see the homepage (index) correctly, however with no images and ALL links to further pages or files are broken giving me an “Internal Server Error” (500) page.
Any ideas on how to solve this?

P.S: I have tested the installation on a differrenet provider (webhoster.de) with no problems. So it must be an Ionos specific problem.

For IONOS, you need to set the RewriteBase in your .htaccess to /.

What changes did you make in that file?

Rewrite .htaccess to / . worked. I was sure I had tried that before.
Thanks a lot.