Unison with Raspbian file server no longer works after upgrading my laptop to Ubuntu 20.04

I have a file server running Raspbian 9, and use Unison over SSH to sync my home dir between these two. This has worked well for the two years I was on Ubuntu 18.04, but after upgrading to Ubuntu 20.04, syncing over SSH aborts with an unmarshaling error, claiming the two Unison versions (local and remote) were built with different OCaml versions. Unison version is 2.48 on all systems involved (pre and post upgrade). What gives?