login insisting that I name sessions

Since I was trying to fix my user list issues with lightdm last week (now resolved), I get a dialogue box after logging in which only gives me two options: log in or name my session. I don't want this, and it will certainly confuse the other users of this computer if it is happening to them.
I am not ticking "save session" on the login screen.

I presume that I have accidentally changed some setting while rummaging in config files - any idea what?

(OpenSuse 13.1, Gnome, XCFE, lightdm)

Richard