[gradualist]

I had the same problem. My Library/Caches folder didn't have the folder mentioned above and I couldn't quit, but I found the rebuild database solution recommended by jimmygster worked for me. HOWEVER! I lost all the changes I'd made since the problems started happening (luckily only earlier today) so if you do this, make a copy of everything you've added recently or try doing a backup.

The problem started happening after I used MacKeeper's cleaning utility (while Omnifocus was running) so I guess that explains it.