So I ended up uninstalling all the Steinberg software (I backupped the Cubase preferences), i.e. everything with Steinberg in the name in Windows Apps settings, plus the elicenser program.
I also manually deleted all the “Steinberg” folders inside my C: drive and reinstalled everything. Then I put the preferences back (but I had to delete the Mediabay files in C:\Users\ [my user name]\AppData\Roaming\Steinberg\Cubase 12_64).
I get the same message - I’m sure the problem is that ‘HALion Sonic SE Content’ was licensed under eLicenser.
I don’t intend to solve it the way Geppastro did - I’m not going to reinstall everything for this harmless but annoying bug. I think I should be able to remove only the un-licensed component in Steinberg Library Manager… if I could figure out what the Library Manager calls it? Any idea which item in Library Manager relates to “HALion Sonic SE Content”?