Hard to say without more information. Possible the app that is updating the files is creating a new version at the time of the backup. Can you provide any additional details on the file types and how they are being updated? For a fast response, consider opening a Support ticket from the Tools - Diagnostic toolbar option.

One file is a .xls file. It is either updated by a manual change, or by VBA code. There is also a second version of the .xls file that is created by VBA from the first .xls file. It is a copy with the date included in the name. The other file is a .dat file. It is normally updated by VBA in the .xls file by adding a record to the end.

I'm going to be updating it in a few minutes, and again in a few hours. It should backup every couple of hours. If it fails, I'll tate your advice and do the Diagnostics thing.

Judging by the issue description, you're using Google Drive.
It's a known problem which is caused by their API. That is actually why we're moving away from this cloud provider soon, since it's not intended to be used for automatic upload operations.
I recommend switching to Google Cloud instead.