Dropbox can't sync because folder is empty.

 I've had this a few times now, and failed to track it down, resorting to things like clearing the .dropbox.cache, quitting dropbox, restarting, disabling selective sync etc.

I thought that the problem was due to a script I run fairly often, which deletes all log files in all subfolders in a project and also pushes changes to boiler plate code to any folder in the project that currently has a copy. I thought that maybe if I ran this too frequently, while a previous sync was in progress, it would just be too much for Dropbox.

What I've just discovered though, is that the blocking folder, which is empty/deleted in Dropbox, still exists on another machine and that a log file in that folder remains open because the Python script that owns it is still running.

I quit the script and all was well once again.

So maybe if you get this error, check that a file isn't open.  If you can't be bothered to track it down, a good old turn-it-off-and-on-again will probably do the trick.