Do you use Thunderbird?
I have a very complicated configuration, including using PGP to sign and encrypt certain emails, and I merely backup/restore the ~/.thunderbird directory.
Now, sometime around version 78 Mozilla introduced some breaking changes. They then skipped a bunch of versions, for reasons unknown to me. But, if your older Mint was using an older version of Thunderbird, it may have broken stuff.
It took me about an hour to get things squared away with the new version, needing to mostly restart from zero. I did have backups for my key files, which saved me some time and provided continuity.
If that's the case, it's probably not Mint's fault. It was gonna happen eventually.