This error is occuring when user changed his username.
You either change username back to original in your joomla/mambo system
OR
identify user ID in your joomla/system for this user, then
1. Delete this user from blastchatc_users table
2. login to
www.blastchat.com, click "Your registration", select your website, click "Users" menu, find same user and delete him.
This will remove old information from your local database as well as from blastchat database, and when next time user enters chat new information will be created for this user.
Why this i snot automatic? Because for security purposes, blastchat is recreating security code of user upon entry to verify that user is who he claims to be.