I think it might be server-side. Earlier this week, I installed XF 2.0 in a test folder off my live board's root so I could tinker with the new features, and I created a new mySQL database exclusively for the test board. I was working with it at around midnight and posted several messages to see how everything looked. Two of them included image attachments. Everything was working smoothly. Meanwhile, my users were posting away on the live board.
I just tried attaching a file to a new thread on my test board. XF 2 stopped the upload and crossed out the file name.
EDIT: Another user just commented that they've encountered the same error a few times before. It seems to resolve itself after an hour or two, so it's apparently a recurring issue. Nevertheless, I'd like to find a permanent solution.