Sorry for the late response, I thought I had already responded a few
days back, but it seems I must have replied to the wrong message...

I'm sad to say that I have had to give up on using Guix (for now), since
I didn't have enough time to properly configure the system to my needs
during university (thanks for the responses anyways!).

Nevertheless, I managed to solve the issue by reading through the
manual, and it was unrelated to my initial suspicion, and as you mention
had to do with the xsession file containing a command (I believe it was
"xset") that I hadn't installed in my user profile. After removing said
lines, any all DE's worked as expected.

Ultimately, I'd still see this as a kind of bug, since the in the case
of GDM no error message appeared at all, and SLiM generated two written
*over* one another, with little information on what exactly was wrong.
Or is it generally not advised to use an .xsession file?

Ludovic Courtès <ludo@gnu.org> writes:

> Hi Philip,
>
> philip@warpmail.net (Philip K.) skribis:
>
>> I've reinstalled the system multiple times with different DE's (LXQt,
>> Mate, XFCE) and DM's (slim, GDM), but the issue persists: The first time
>> I try to login, it works, everything is loaded the way I'd expect it,
>> but as soon as I install anyting (using "guix install ...") and *then*
>> log out, and try to log in again, the screen just blinks and throws me
>> back into the login screen of my display manager.
>>
>> I've tried changing /etc/config.scm and keeping it the same, reveting
>> package instalations, but nothing seems to change anything.
>
> Like Giuseppe wrote, could you share your config.scm file?
>
> What happens if you remove ‘xmonad’ from your profile?
>
> Do you have a ~/.xsession file?  If so, could you share it?
>
> I don’t see any reason off-hand while this would happen (Guix System
> installations used SLiM in the past, and have been using GDM by default
> for several months now.)
>
>> When running "startx" in a TTY, I get this message, no matter what WM I
>> try to use:
>
> Indeed, ‘startx’ is currently not supported.
>
> Thanks for your bug report,
> Ludo’.

-- 
	With kind regards,
	Philip K.