I just recently began receiving the same error message reported at the beginning of this thread. QB does not backup my file although the file seems to be working properly.

Some information: QB V18.0.13 R14, macOS Big Sur 11.2.3, iMac 2020

I have tried many things:

One thing I have found is that when I locate the company file in the Finder and duplicate it, it works just fine in QB. So I can make a manual backup.

Another, I created a new simple company file with just a standard chart of accounts with no data and got the same backup error message when attempting to backup.

I have an older Powerbook running QB V18.0.9 R10, opening the same data file (stored in the cloud), and backup works as always.

I deleted and reinstalled QB on my iMac - same error message.

My conclusion is that the most recent update of QB is what is causing the problem. I'm certainly not going to upgrade my Powerbook until this issue is rectified. Can anyone else confirm my experience?