Welcome to the forum @DamianTMB
On Windows, there have been some reports where an antivirus deleted the file, but there are others that didn’t fit that. The error where Duplicati SpillCollectorProcess is missing a temp file in FileStream.Init has not been isolated, as far as I know, and one reason might be that people post the ending, with no context.
Duplicati might also not produce a backup log when a backup doesn’t complete. If you have one, it’d help.
I don’t know if it’s possible to make a smaller test backup that reproduces the error, but that’d help as well. Logging becomes more feasible, e.g. –log-file and –log-file-log-level=retry, then either paste that here with three backticks (```) above and below, or zip it and post it as a file (if it winds up large despite your efforts).
EDIT:
might be relevant, if Azure has quotas or similar limits. You might see some retries in the retry log, which you can also watch at About → Show log → Live → Retry, but that’s not very good for posting to forum.