I have found a fix for this. I have a Nokia Lumia 710 and on T-Mobile. I tried everything.
In the end it worked by switching the phone off, removing the battery and sim card. I then turned the phone on with the battery back on, but without the sim. I turned my WIFI on, then logged into chat by going online, took a while but came on.
It also had reset my whatsapp live tile, which was stuck at 99 messages as it would not reset. Logged into whatsapp, then logged back out and it was cleared.
Switched phone back off then, remove battery, and reinstall sim, then put battery back and turned phone back on.:evil: