Couldn't login

[O Bedford uni-koeln de]

This morning when I tried to login via gdm X showed only a grey
background a the default X-cursor (no windowmanager, no desktop, no
panel).

.xsession-errors contained:

Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key
freetemp 1.31 - could not open the display
Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key

Gtk-WARNING **: cannot open display: :0
SESSION_MANAGER=local/triton.pc.uni-koeln.de:/tmp/.ICE-unix/15183,tcp/triton.pc.uni-koeln.de:1462
Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key

Gtk-WARNING **: cannot open display: :0

  After removing .ICEauthority the login suceeded.

  What's causing this? And how do I prevent it from happening in the
future?

  Oliver