I'm not sure if do-release-upgrade supports skipping releases, except for going from lts to lts? In other words, it might either: 16.04 -> 18.04 -> 20.04, or 18.04 -> 18.10 -> 19.04. But not 18.04 (lts) -> 19.04?
Sorry, brain fart – you have to upgrade between non-LTS releases sequentially, as far as I know there's no supported way to upgrade to 19.04 from anything but 18.10. (in general, to get to release n, you must either be on release n-1, or n must be an LTS and you must be on the previous LTS.)
Sorry, totally missed that. Looks like some other people have had similar issues with upgrading to previous versions: https://github.com/Microsoft/WSL/issues/3489 (though the issue is for (I think) 16.04->18.04, someone mentions that it also affects 18.04->18.10)
it's strange that that's all the output you get, though. Apparently it puts logs in /var/log/dist-upgrade/, is there more there?
AHH, thank you! /var/log/dist-upgrade/main.log explains why! It's indeed another instance of a bug I already saw when upgrading another system [1] and couldn't figure out -- there's a missing dependency in some Ubuntu package: