OK, “Unexpected Problem” popped up, and here I am (a few minutes later)…
System: macOS 10.15.7 Catalina (still!), Jaikoz 11.6.2 Pulp, foobar2000 (mac) 2.8 beta 8.
I had 155 songs loaded-in, filtered on one artist, selected one row and did [File > Play Song] which appeared to trigger this.
Just before, I had selected and played a different track, and my default player-app opened, namely foobar2000-macOS, which played it correctly, after which I quit the player. Then I copy-pasted ( [Cmd-C/V]) a genre value to that track. I’m not sure (I got woken early!) but possibly I accidentally clicked the wrong hotkey just prior to [Cmd-V] - no idea which one.