Cannot login into XFCE after logged out of GNOME because SESSION_MANAGER env is set.
If you login to XFCE with gdm and then logout to login into Gnome and then logout to login again into XFCE it fails because XFCE says that another session manager is running(the environment variable SESSION_MANAGER is set). This also happens if you login to Cinnamon and Mate instead of Gnome, but not with LXDE and KDE Plasma.
I've tested it in Debian Stable (GDM 3.22.3, XFCE 4.12) and Arch Linux (GDM 3.28.1, XFCE 4.12).
Logs: journalctl.log dbus.log