I think I experienced the same when I was test running WR 4.4.2 and it did the auto registry backup prior to opening to the repairs.
It said there was an error with an additional message that the program hadn't shut down properly which I didn't understand.
I shut the program down, cleared the cache with CCleaner, rebooted and ran the repair program again where it successfully created the registry back up.
On checking the back ups, the successful one was 149MB with what I assume was the failed one at 86MB.
This was on 1809 so the OS isn't relevant but it's the first time I've seen that error.