Comment # 9 on bug 1085772 from Fabian Vogt

It appears to me like this is a misunderstanding.
gdm already runs a dbus daemon for the session, but plasmawayland.desktop
doesn't expect that. Instead, it contains:

Exec=dbus-launch --exit-with-session /usr/bin/startplasmacompositor

It's confirmed by https://bugzilla.gnome.org/show_bug.cgi?id=760613
However, it looks like the reporter never told upstream about this so it was
never actually looked at.

Try to replace Exec= in /usr/share/wayland-sessions/plasmawayland.desktop with

Exec=dbus-run-session /usr/bin/startplasmacompositor

or, if that doesn't work,

Exec=/usr/bin/startplasmacompositor