I had this problem, and I posted the results in this forum. Ancestry Support suggested deleting the on line tree and resyncing, which I did, but the problem remained. I then ran the Sync Troubleshooter which is available in the FTM files on your PC. This pointed to a single .jpeg file, so I deleted that from the person in FTM, synced and the error cleared. I then copied a new version of the same file into FTM and it synced OK.
Take a look for my previous forum posts on this problem, should help you !