Changing user only half-works

[Snark]

Version: 1.2.1-0ubuntu1.1

Launch a session (unity), then ask to change user to a specific user : ok.

Launch a session(unity), then ask to change user without specifying which : it doesn't work. The screen becomes black ; changing VT allows to get back to the existing session(s), but no chooser comes up. In fact, I think the X server gets up -- the log looks like a normal start until ddxSigGiveUp: Closing log, Server terminated successfully (0). Closing log file. -- but it shuts down immediately, and I don't know why.