I'm having problems too and it's the first time I've had this issue. I'm using FTM 12 with Windows 7. I've tried a manual high level check on records and replicated some record merges I'd previously made on FTM on Ancestry to bring the files more in line. I left it overnight but it's still failing this morning towards the end of the Sync when downloading form Ancestry. I'm going to try powering down my laptop in case there is a a cached value or signal that's stopping the sync. After that I'm stumped and may have to make a new copy from either Ancestry or FTM - but I'm not sure which.