"Since when you have this issue? Have changed PHP version, added extra rules to .htaccess or something else?"
Sorry for not responding. I have this issue for several weeks / months now, still trying to find out what goes wrong.
I had not changed PHP version (was running 7.4.24) but I did tonight (8.0). This did not solve the problem.
I didn't do anything with .htaccess
I see the problem is now solved, but it looks there's another part of CSS now missing, because other things are not working correctly. For example my logo at the top left corner, that's now on a white background, that should be a dark background. So there's something missing. What have you changed? We're getting to a solution ;-)