Unison and version/compiler conflicts

I have two machines that one runs Ubuntu 18.04 and the other one Ubuntu 20.04. I want to use Unison to sync files between them but there are some issues. Firstly, because I sync with another machine too, I compiled from source Unison 2.48.3. Then in the Ubuntu 18.04, I installed from the Ubuntu software Unison 2.48.4 and so I did with Ubuntu 20.04. After setting them up properly, from the command line of Ubuntu 18.04 I run

-2.48.4-gtk

and try to sync. Although everything works fine, i.e. it finds the machine and the directory then I get the message:

error during unmarshaling (input value:ill-formed message) possibly because client and serer have been compiled with different versions of OCaml compiler

. In the 20.04, I checked the version of OCaml but apparently it returned nothing (even though I had installed unison from app store). On the 18.04 the version is 4.05.0. Any ideas on what I can do?