Comment 17 for bug 1568459

For me on Ubuntu 19.10, the best workaround seems to be to copy its newer unison-2.48.4 to my old Ubuntu 14.04 machine (and put it into /usr/local/bin/). Then on my 19.10 machine, I simply add this line to the *.prf config file I'm using:

  servercmd = unison-2.48.4

Alternatively, the ancient Unison binaries do still work on Ubuntu 19.10 (yay!), so I can run those when necessary to talk to an old stock system.

More details on the specific combinations that seem to work vs. break:

Ubuntu 19.10 appears to run these ancient binaries just fine:

- unison-2.40.102-gtk, from the Ubuntu 14.04.4 LTS package
  unison-gtk_2.40.102-2ubuntu1_amd64.deb;
  it talks to the stock unison unning on Ubuntu 14.04.4.

- unison-2.32.52-gtk, from the Ubuntu 14.04.4 LTS package
  unison2.32.52-gtk_2.32.52-7ubuntu1_amd64.deb;
  it talks to the stock unison running on Ubuntu 10.04.4 LTS.

Ubuntu 14.04.4 appears to run this NEWER binary fine:

- unison-2.48.4, from the Ubuntu 19.10 package unison_2.48.4-1ubuntu1_amd64.deb

Ubuntu 10.04.4 LTS will NOT run these NEWER binaries at all:

- unison-2.48.4, from Ubuntu 19.10.
- unison-2.40.102, from Ubuntu 10.04.4 LTS.

$ unison-2.48.4
unison-2.48.4: /lib/libc.so.6: version `GLIBC_2.15' not found (required by unison-2.48.4)
unison-2.48.4: /lib/libc.so.6: version `GLIBC_2.14' not found (required by unison-2.48.4)
unison-2.48.4: /lib/libc.so.6: version `GLIBC_2.17' not found (required by unison-2.48.4)

$ unison-2.40.102
unison-2.40.102: /lib/libc.so.6: version `GLIBC_2.15' not found (required by unison-2.40.102)
unison-2.40.102: /lib/libc.so.6: version `GLIBC_2.14' not found (required by unison-2.40.102)