Are you by chance using CloudFlare... and did you by chance enable Rocket (EXP)Loader?
Do yourself a favor.. never be tempted to enable Rocket (EXP)Loader, no matter how beneficial it sounds. For several years it has had a tendency to cause various issues on XenForo. ;)
xenforo.com
Another issue could very well be that your host has mod-security enabled and has NOT updated the rule-sets. There WAS a bad rule-set recently that caused issues with certain XF PHP files... and I think that
css.php
was one of them.