If would have nice if someone had posted this, but I didn't have time to contact support and find out what they said. It's pretty clear that the problem is a lock file that got left around. A proper system would notice that it was stale (it's not going to take an hour to synchronize) and see if it was really locked, but that doesn't seem to be happening here.

Quit OmniFocus.
Go to ~/Library/Application Support/OmniFocus/Synchronization Data/Run
Move the contents of that folder to the trash.
Restart OmniFocus

Mine was happily back to where it should be, hopefully yours will be too.