Not sure if you're still having this problem, but if you look at server.xml (located in "<yourinstallationfolder>/server/etc/groups/default/"), you'll see the two elements below. Keep in mind though that you'll need direct access to your 123flashchat program files. If you're on a shared server chances are you don't have direct access to them and you'll need your host to edit these files for you.
<integrated-other-database>
<auth-url charset="UTF-8">
these both need to have the correct values. The correct values are:
<integrated-other-database>URL</integrated-other-database>
and
<auth-url charset="UTF-8">http://yourforumdomain/vbb.php?username=%username%&password=%password %</auth-url>
After the values are properly set, your flashchat server needs to be restarted. If you don't have direct access to restart your flashchat server then you'll need your host to do it for you. If you do have direct access to SSH, then login and the command is:
./fcserver.sh restart
You'll of course have to issue the cd command to be in the correct directory (mine is /usr/local/123flashchat/server/)
Hope that helps