There were a bunch of dangling images. Deleted those. Update failed because you no longer use AWS (....lol), but after https://forum.cloudron.io/post/59939
Update is complete. Well, 5 or 10 updates. Hey, everything kept working for years...!
Surfer installed successfully. Thank you for your help!
Note to anyone doing this in the future: the updates will result in a lot of backups (which can be removed via the web interface or at /var/backups) and a bunch of dangling or unused docker images which can be removed with
docker system prune
or
docker system prune -a