@Schlecht I had this problem at one point. The only way to found to fix it was to delete every cookie related to .xbox.com and .live.com. I believe that the Live sign-up page had changed and didn't migrate my cookies properly, or my cookies got corrupted, or something like that.
Xbox Live Beacons and other new social features incoming to Xbox.com
Nov 9th 2011 1:17AM (Joystiq)Twin Peaks receives fan-made Atari 2600 game 'Black Lodge'
Sep 27th 2011 8:25PM (Joystiq)