Comment # 1 on bug 1091818 from Neil Rickert

I'm surprised to see this.

I ran into this problem a while ago.  But it is no longer happening for me.

When I first saw the problem, after login to either Plasma-wayland or
Gnome-wayland, any attempt to login to Plasma-X or Gnome-X would fail (return
to login screen).  I have not had that happen for a while now.  Perhaps it
depends on graphics card.  I seem to recall having this problem with both SDDM
and with GDM.  And I could still login it Icewm.

The workaround that I used: CTRL-ALT-F1 to get a command line.  Then kill any
processes running under my userid that were older than the current login. 
Usually killing the "systemd" process was enough to take down the rest.  After
that, I could login to X sessions with Plasma and Gnome.