I have had this problem off and on. I currently cannot sync my tree. Even after a full successful re-download of my tree, the next time I try to sync I get that mysterious message. I have to re-download my trees every couple weeks now if I want to use FTM. Even my small trees have started failing.
You should contact Ancestry support and they will give you a list of things to try first. If those are not successful, you will have to re-download your tree. That may or may not allow a successful sync after that. Last time I talked to them, they asked for patience as they are still working on the sync issues.
Don't sit back and wait for it to get fixed. Send them information about your errors so they know that it is still an ongoing issue. They may have you run a Sync Reporter feature, which gathers information about your tree and your computer and the failure.