Comment 25 for bug 1568459

This may help.
After upgrading my Desktop from Mint19 to Mint20 I could not get unison
file sync working to my Ubuntu Server. With some help from the guys on
the GitHub issues list I have now fixed it. Apparently unison for
Ubuntu19 and Mint20 are compiled differently, and even though they are
both version 2.48.4 they are compiled with different releases of OCaml,
which makes them incompatible.
This is what I did -
Remove all traces of unison
then run
wget http://mirrors.kernel.org/ubuntu/pool/universe/u/unison/unison_2.48.4-1ubuntu1_amd64.deb
wget http://mirrors.kernel.org/ubuntu/pool/universe/u/unison/unison-gtk_2.48.4-1ubuntu1_amd64.deb
sudo dpkg -r unison this removes the old install
sudo dpkg -i unison_2.48.4-1ubuntu1_amd64.deb
sudo dpkg -i unison-gtk_2.48.4-1ubuntu1_amd64.deb
sudo apt-mark hold unison this stops it updating
sudo apt-mark hold unison-gtk

This is not a proper solution it is just a fix, but it did get me out of a hole :)
Mick