[Andrew]

To elaborate a bit, if you are seeing this and it is because you have multiple devices syncing simultaneously (two Macs, or a Mac and an iPhone, etc), then the "correct" way to handle it is to choose the Cancel option, wait until the sync on the other device is finished, then you can attempt the sync again on the device that reported the problem.

We believe we have a fix for this, which is already in the latest sneaky-peek releases of the desktop app, but will still be a problem until 1.0.2 for the iPhone is available (sorry, I don't have an ETA on that).