The problem was that rexml have been separated from ruby. I've added that as runtime dependency.
I don't know why ruby-rexml is marked in red on this page. The package is certainly there and installable:
https://archlinux.org/packages/community/any/ruby-rexml/
@Muddy: I think you are right there are possibly something not working with Gnudb. MusicBrainz is working in GUI atleast. But I am currently sick and don't have the energy to look into this right now. So I don't know really. However there's a bugreport you could look at:
Pinned Comments
FabioLolix commented on 2024-10-19 12:37 (UTC)
After every major
ruby
update all AUR ruby pkgbuilds need to be rebuilded