I believe the sync error has to do with compacting the file. The FTM works beautifully until you compact you tree. This is a software problem and needs to be fixed immediately. By the threads I could find on this subject, it appears that Ancestry.com either hasn't heard about the problem or is having a difficult time resolving the issue. There are people who have been posting for a very long time on this topic... many threads out there. There's no way that I can unlink my tree with success. Given the choice, there would have to be two trees. One for FTM and one for Ancestry.com... what a chore!
By the way, how do we un-compact our tree???
Also, the FTM shuts down on certain requests. The theory is great. The ability to attain successful results is difficult.