Comment 10 for bug 1868502

In fact, further investigation shows that the binary built with the OCamL 4.05 compiler is working fine. The error message quoted in my previous post is obviously caused by a cache file corrupted by previously running the binary from deb (compiled with OCamL 4.08). Deleting the cache file fixes the problem.

I have now verified that just using the binary copied over from Bionic works equally well on Focal.

So the workaround seems to be to uninstall the unison deb package and to copy the /usr/bin/unison-2.48.4 binary from a Bionic system into /usr/local/bin, /opt/bin, $HOME/bin or whatever else you prefer (and possibly to create a symlink called unison).