Final outcome:
After I downloaded all my files from Backblaze, I unpacked them and the slash at the start of each file name was, as expected, automatically removed by windows.
I uploaded all files to a new bucket and set it up with “B2 Cloud Storage” in Duplicati this time.
I rebuilt the database and was greeted with a new error.
15 x
[Error-Duplicati.Library.Main.Operation.RecreateDatabaseHandler-MissingFileDetected]: Remote file referenced as ... by ..., but not found in list, registering a missing remote file"
As discussed here it is likely due to a failed backup in the past.
I did what the other user did and deleted all the these files. They were all from 2022, so not randomly across the years I used this backup.
First I did this in a local copy of the backup. After the files were removed (renaming while messing up the extensions does not work) no errors on repair, or test or verify or restore. (Minor warning due to a restored file being open during restore).
Did the same in the B2 bucket, without a full restore, also no errors.
Updated Duplicati, run a backup again, no errors.
So, now I’m back to a running backup with the current version.