As other users I began getting this error yesterday. I did all the recommended steps short of unlinking my tree and was confident that the file was corrupted and I would have to start a new sync tree online.
Husband suggested a fix.
This will only work if you are running Windows 7. Shut FTM - open up documents and search for your working directory of your FTM file - right click and chose "restore previous version" chose a restore point when the file worked and click restore.
Re-open FTM and your file should sync ok (or mine did). Hoping I haven't jinxed myself!