Comment 19 for bug 1868502

To make it work, I had also to replace unison-gtk, not only unison. So, what finally helped was:

Problemsituation: I had updated laptop A to a newer version of ubuntu. When trying to synchronize with with laptop B (Debian), and therefore did start unison-gtk on lapotop A, I got the error we discuss here.

The solution that worked for me:
1. On Laptop A: downloading from packages.debian.org exactly the unison Version *and* the unison-gtk version, I have in use on Laptop B
2. On Laptop A: Installing both:
sudo dpkg -i unison_2.48.4-1+b1_amd64.deb
sudo dpkg -i unison-gtk_2.48.4-1+b1_amd64.deb
3. On both, Laptop A and Laptop B: Removing in the hidden folder .unison all files, with a filename starting with "fp...", that are created after I had updated laptop A to a newer version of ubuntu.

To make sure, Laptop A wont "upgrade" unison and unison-gtk I created, like mentioned above, a file with filename "unison" in the folder /etc/apt/preferences.d/ wrote this text into my file "unison":

     Package: unison
     Pin: version 2.48.4-1+b1
     Pin-Priority: 1000

     Package: unison-gtk
     Pin: version 2.48.4-1+b1
     Pin-Priority: 1000

Thanks to all here for those posts that helped me to do so!