I'm also having the exact same issue. I try to sync and I get the error message:

The last sync failed. An error occurred while "Analyzing online changes". You may want to restart FTM and try again later.

I called Ancestry.com and the only solution the tech could offer was to tell me to unlink my FTM from my tree and upload my FTM files as a new tree... at the cost of many hours of time and tons of file transfers and bandwidth. The Ancestry tech on Facebook isn't much help either, at this point.