Happening to me too just after the Wheezy -> Testing upgrade. No additionnal clue of what's causing this, I installed i3 meanwhile :D
Joining .xsession-errors.
Xsession: X session started for fab at lundi 6 mai 2013, 20:29:20 (UTC+0200) localuser:fab being added to access control list X Error of failed request: BadWindow (invalid Window parameter) Major opcode of failed request: 12 (X_ConfigureWindow) Resource id in failed request: 0x0 Serial number of failed request: 629 Current serial number in output stream: 633 thunar: Fatal IO error 11 (Ressource temporairement non disponible) on X server :0. Error: cannot open display: :0
