!CreateDirectoryRecursive(fullpath)And there were two buttons: one said "Quit", and did nothing. The other said nothing, and did nothing. To exit Unity I'd have to kill or ctrl-c if I started it from command line.
This went on for quite some time. I even tried re-installing, and deleting every configuration file I could find. I searched the internet, but the only one similar report I found on this error was not helpful.
So finally I straced the thing and found the problem... ~/.cache/unity3d was for some reason owned by root.
Simple, but amazingly frustrating to finally track down.
(I also installed unity on Arch linux using AUR, and it worked fine with no such complications.)