I have hit the same issue. I changed the MIAB backup today from using S3-compatible over to AWS S3 and it all stopped working. Running a full backup manually fails with that -gpg-options error. duplicity is v20. I deleted /home/user-data/backup and reinstalled the latest MIAB, did not help. Just found this thread so I downgraded duplicity to 0.8.21 and now my manual full backups get further - no module named ‘boto’. I have seen that somewhere in these threads, will chase that down.
Thanks for posting 
Update: ran apt install python3-boto and looks like backup is running. Clean install of boto, was not needed before??
Update #2 But now I cannot get the AWS S3 details to work… aargh