So, I've seen a couple threads sounding the same as this. I am in the same boat. I can't exactly help you, but I have found a possible reason for the issue. Mine also did this after I linked some of my contacts to their facebook profiles.
So as an experiment, I created two new family trees. One I created on Ancestry.com and another I created on my computer using FTM.
First I created one on Ancestry.com, then opened FTM 2012 and downloaded that tree to make a new file on my computer. All was syncing well, until I went back to the version on Ancestry.com and linked it to Facebook. Then on my next sync, it downloaded the changes but then said sync never finished. On subsequent syncs it shows me the same potential changes, but doesn't sync them again. It just finishes with a sync error.
Second, I created a new tree in FTM and uploaded it to Ancestry.com. Again, all was syncing well until I linked the people in the tree to Facebook. Then the same errors occur. So now, what I need to figure out is how to break that link to facebook. Hmmm. Anyone from Ancestry.com out there to help?