Yes, but I suspect this is a Joomla / server issue due to some block of the logs-folder so I always had the error-500-issue. The site obviously can't be located in root.
I just copied the whole website into a folder called "test" and now everything is working perfectly. However, this is just the test-page. When I copy the actual page once again in another sub-folder, then the password-issue is solved as well, but now many pictures are not shown "access denied", but all folders are set to "writable".
So, one issue solved, another one came up :ohmy: