KDE Switch user causes crash

[trjkl]

Hi,
I'm having trouble with switch user in KDE.
When I select switch user from the menu, a new login screen appears as expected. After selecting a user and supplying the password, i get the usual progress bar but it stops almost immediately and nothing else happens.
I then have to resort to the reset button.
I can login, logout and login as another user without trouble and right up to the point of submitting a password, i can switch betweeen sessions (is that the right word?) using ctrl+Alt+fkey
I'm using the default/linux/amd64/17.0/desktop/plasma (stable) profile
I found this topic https://forums.gentoo.org/viewtopic-p-8173788.html and have tried the suggestions of using elogind instead of consolekit.
I've also found something that suggested that the problem was fixed in a later version of dbus so i now have the followiong keyword =sys-apps/dbus-1.12.8 ~amd64
The behaviour is the same.
I've also tried lightdm instead of sddm, interestingly, that doesn't even allow the first graphical user to log in so I've gone back to sddm.
There doesn't seem to be anything in syslog or sddm.log that suggests what's going on.

Any ideas on where to look would greatfully received