Unison in 20.04 won't talk to older unison

[Jeff Abrahamson]

I'm running unison 2.48.4 on two hosts. One I just upgraded to 20.04. The other is still at 18.04 LTS. The two unison instances no longer talk happily together. The error is this:

Fatal error: Server: Fatal error during unmarshaling (input_value: ill-formed message),
possibly because client and server have been compiled with differentversions of the OCaml compiler.

which is repeated many times.

It seems quite surprising that the issue is really only resolvable as "upgrade all unison instances that might ever talk to each other together or not at all", so I suspect the error message about OCaml build is a red herring. I'm not, however, sure how to debug this, however.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: unison 2.48.4-4ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-33.37-generic 5.4.34
Uname: Linux 5.4.0-33-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.2
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: i3
Date: Thu Jun 4 21:35:59 2020
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-10-23 (1686 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
SourcePackage: unison
UpgradeStatus: Upgraded to focal on 2020-06-04 (0 days ago)

[Jeff Abrahamson (jeff-purple)]

This may be a duplicate of #1868502 which is potentially related to #1568459.