After messing around with it for a bit, I found that the disk doesn't work with "EmuTOS" that Hatari uses by default. If I instead use some other tos (eg 1.02) then it seems to work fine.

I find it a bit weird that the dbug menu seems to just ignore keypresses when using EmuTOS, (rather than trying to run the game and failing or whatever) - but I guess that's not very important. It just made it a bit harder for me to guess the problem!