I overlooked this topic when searching. I had the same problem with the “cannot open file” after an update.
Apparently the update threw away my own cat_colors.css. I’m 100% certain I had that file at that location b/c I entered the colors manually in that file. Replacing it with the original or a blank one didnt work.
So file permission it was (755 wasn’t enough). All 777 did the trick, but I’m not too happy about that…
Some observations:
I fiddled some more with the permissions. And finally (somehow) all files and folders geting 755 /was/ acceptable for the site to stay up. Weird…
//Update: maybe my cache was playing tricks on me…
