Comment 16 for bug 1868502

So I also found a workaround that works for me so far. It is also downgrading to the bionic versions.

First of all, I downloaded the packages from bionic and installed them manually (only unison and unison-all, though) as suggested by Sebastien Koechlin and Helge Meinhard said. Then, I created a file to pin it to that version so it will not get upgraded, i.e., in /etc/apt/preferences.d/unison I have:

    Package: unison
    Pin: version 2.48.4-1ubuntu1
    Pin-Priority: 1000

    Package: unison-all
    Pin: version 2.48+2
    Pin-Priority: 1000

The other problem is that unison does not like to use the old archive files. You can try running unison with the `-ignorearchives` flag. If unison runs then, you have to remove the old archive file.

If you have only one archive, you can just delete your ~/.unison directory and then run unison afterwards without any flags and it should be fine.
If you have more archives and you do not want to rebuild them all, you have to find the proper file.
I did this in a stupid way by copying them out one-by-one (both the arXXX and the fpXXX).
The, I check if unison still reports an error and if so, it was the wrong archive and I copy it back.
If I found the correct archive, I just stop.
Maybe there is somebody who knows a flag which can tell you the archive file directly.