Back on 17 Dec 2012 I had the same thing happen with FTM2012 and my online tree. Kept trying to sync and then had to do a quick cancel when info being brought into FTM was being corrupted. Finally bit the bullet and unlinked FTM and my AMT (Ancestry Member Tree). Ever since have been unable to download my AMT into FTM. Fails about an hour into the download. About two weeks ago I even stopped being able to generate a GEDCOM. It quits after 48+ hours at the 65% complete mark. Today I called Customer Support and a ticket entered about the GEDCOM issue and, to address the sync issue, I was asked to submit my last Synced file (an ftm backup file from 18 Dec). Because it's 34G I was told to connect to http://www.ge.tt/
site to upload the ftmb file. Only problem is that I can't connect to that site using any of the four major browsers (IE8, Safari, Chrome and FF). Am now between a rock and a hard place.