Graphical login broken on core-updates

[Andreas Enge <andreas@enge.fr>]

Hello all,

first the good news: On my x86_64 machine, most of what I need is available
from core-updates. I just updated my system and one user. This includes
- xfce4 (gnome also builds)
- openjdk
- ghc-pandoc
- calibre (with qt@5, python and python-pyqt)
- unison (so also ocaml)
- icecat
- ungoogled-chromium
wine cannot be compiled, which reflects the worse state of i686 compared
to x86_64, which is an area of work for the weekend!

The first annoying thing: locales are broken, mutt does not show accented
characters any more.

Worse:
I cannot log in to xfce4 with gdm: gdm starts, I can enter my user name and
password, then I end up with a black screen with a graphical mouse pointer
that reacts to moving the mouse, but nothing more. Ctrl-alt-f1 followed by
ctrl-alt-f7 brings me back to the gdm login screen. I tried to delete a few
random directories in .config and .cache, but to no avail; maybe they were
not the right ones... I also rolled back just the user profile, which was
also not enough. During this time, logging into xfce4 with a second user,
whose profile was still on master, was possible.

I could "solve" the problem by rolling back both the user and the system.
So at least the Guix killer feature still works ;-)

Do you have an idea what is happening? Is there anything I can do? Are
there bugs in core-updates? Is there anything we should tell users before
merging? Thanks for your enlightenment!

Andreas