Okay so I checked all the permissions and there was no problem.
I backed up the entire “public_html” directory into a gzip and then unzipped a new Joomla install in there.
Unfortunately, I got the same “400 Bad Request” error
So I deleted everything the directory again and added 1 file – phpinfo.php with the appropriate code and permissions.
No dice – still getting the 400 error.
I will send a ticket to the provider and see what they say.