Sophos blocks SuperDuper due to failure to delete threat

I have been using both Sophos for Mac and SuperDuper for about 7 years. Until fairly recently, they were able to work together fairly well. But a couple of months ago a problem has developed which blocks SuperDuper every time, required much manual scanning, manual cleanup, and rerunning SuperDuper. The root cause of the problem is that Sophos quarantines threats, and when SuperDuper tries to copy them, access is denied and SuperDuper terminates at that point.

Here are the details of the problem:

1. SuperDuper is set to run nightly.

2. Sophos is set to "delete threat" in all scans.

3. During a SuperDuper backup, a spam email is detected and it is quarantined.

4. The threat is not deleted - it just stays in the quarantine.

5. SuperDuper finds that entry in the quarantine or somewhere, and terminates. Since SuperDuper makes an image backup, just being quarantined means it will be denied access,.

6. I have to manually run several iterations of cleanup, scan, re-run SuperDuper. Sometimes it takes three or four repetitions of the iterations.

Why cannot Sophos not delete the threat completely and not quarantine it with the need for manual actions. This issue used to happen very rarely. Now it happens daily, and every morning I have to go through this process.