After dist-upgrading a system from Devuan Daedalus using
the Excalibur repos, my X session (mostly XFCE defaults via
task-xfce-desktop) kept dropping me back into slim a moment
after the desktop had shown up.

As it turned out, the culprit was xscreensaver, as packaged
by Debian upstream, with these messages in the user's
.xsession-errors:

> screensaver: 20:34:36: setpgid 3481: Permission denied
> X connection to :0.0 broken (explicit kill or server shutdown).
> X connection to :0.0 broken (explicit kill or server shutdown).
> xscreensaver-systemd: 20:34:36: X connection closed

I don't know why the (optional) xscreensaver-systemd
helper is started here, or how to prevent that - or if
doing that would help at all... So at this point this
mail is merely a hint to people running into the
same problem... For now, I uninstalled xscreensaver.

Alex.