Comment 9 for bug 1875475

The accepted work-around is to manually install prior versions of unison and unison-gtk. It seems to me that the solution to this bug is to replace the faulty unison and unison-gtk in the repository with the known good versions (from 19.04 for example). People who have been using the faulty version from Ubuntu 20.04 would be updated to the good version, after which they would get an error message from Unison when they used the good version with a sync that was previously done with the faulty version. The error message simply tells them to delete the sync archive file and then re-sync.