sorry I've already solved this for myself but will post the solution in case anyone else has the same problem in future:-
Although the error from PlayonLinux doesn't mention it, this is really an error with wxwidgets, which I had previously compiled from a tarball. My vague understanding is that software compiled from a tarball is tailored to the specific local PC, and so it makes sense to me that changing from 20.04 to 22.04 would mean this wxwidgets might need to be uninstalled and recompiled.
To uninstall the old wxwidgets, I had to go back into the "source" folder - i.e. the folder where I had unpacked its tarball several years ago - and run:-
$ sudo make uninstall
(other guides say run make uninstall but ubuntu I think does not use this "run" )
After this step, PlayonLinux began loading as normal again. But perhaps I will still want wxwidgets for something so I next downloaded the new tarball for wxwidgets version 3.2.1 from
and installed it using
$ sudo ./configure && make install