Close all programs where the family file might be opened (add-ons, Legacy FamilySearch and Legacy Charting). Then give the backup another try.
On a few occasions, no add-on programs nor Legacy FamilySearch or Legacy Charting appeared to be opened, but the Windows Task Manager showed they were still running. Closing them in the Task Manager solved error 57.
There is another possible reason. If you have a media file of any sort open in another program while doing the backup you can trigger this error. Close other programs that may be using one or more of the media files.