XF 1.5 Failed to load resource: net::ERR_EMPTY_RESPONSE for css.php

An empty response like that is almost certainly indicative of a PHP crash. Unfortunately, they're very difficult to debug, especially if they're intermittent.
 
Sort of. If you disabled add-ons and sat there refreshing that URL, if it's consistent, it would crash regardless. This probably isn't directly affected by add-ons beyond an add-on triggering the situation that caused the issue.

If I had to guess, it's probably more related to CSS minification, if you have that option enabled in XF. There could be a regex triggering the issue within that. If it's not consistent though (with a given URL), then that makes it super difficult to debug; it might just be something in the server environment.
 
Without a consistent reproduction case, you likely need to disable the option in the control panel and see if that makes any differences.
 
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