I’m sorry, I forgot there are different appearances, I’m using XfCE.
Meanwhile I found this article:
article: Black Screen Desktop on Login, Apr 2021
(I’m not allowed to post hyperlinks, I’m sorry)
I don’t know, how I should have killed the desktop process like the issue says, maybe it relates to updating via pacman -Syu (don’t know much about these things).
However, the command
xfdesktop --replace
works temporarily until the next start.
Deleting $HOME/.cache/sessions seems to have worked even though I have no idea what the problem was.
Thanks for your efforts and sorry again for missing information, next time I’ll do better!