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.  ;)
				
					
						
							 
						
					
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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.