The message went on to say "unfortunately, you will need to unlink and relink your tree".
That, of course, is a disaster.
I tried several things that did not work. Including calling ancestry.com. They sent me some tech notes that wasted my time and did nothing.
I finally found a solution that worked!!
I have been using Crashplan to backup all my files to the cloud.
That means I had multiple versions that I could restore.
Restoring my .ftm file (36 GB) did not work. I tried several versions.
Finally, I looked in the file system folder and noticed some .sync files.
I restored the .sync file from 2 days ago.
I was then able to synchronize!!
Apparently, the .sync file had become corrupted and replacing it with a prior version solved the problem.