I have experienced much the same issue: I leave an X session to run as another user on another VT, open X for that user, exit X, switch back to the original session and it crashes. The log shows:
II) AIGLX: Suspending AIGLX clients for VT switch
[dix] couldn't enable device 9
[dix] couldn't enable device 10
[dix] couldn't enable device 11
[dix] couldn't enable device 8
[dix] couldn't enable device 6
[dix] couldn't enable device 7
(II) AIGLX: Suspending AIGLX clients for VT switch
(EE)
Fatal server error:
(EE) EnterVT failed for screen 0
(EE)
(EE)
Please consult the The X.Org Foundation support_
for help.