It's good to see an integrated VB module, although it would be better it it worked properly! For those who host your own server, but have a license (correctly I believe) for the domain where you have the client, ie. where your users are chatting, and these server and clients are not the same, you will need to fix something.
The integrated widget doesn't give you the possibility of adding different server and client locations. If you enter my_chat_server.com for your server, then the system will also look to 'my_chat_server.com' for the client. That results in an 'access from my_chat_server.com not permitted' error when you try to connect to the chat. I assume because your 123chat license is not for my_chat_server.com, but for your website.
The solution - quick and dirty - is to edit 123flashchat_admin.php and hard code in the URL to the client (line 58):
Code:
$fc_row['fc_client_loc'] = 'http://www.my_actual_website_not_my_chat_server.com/123chat/';
Thanks to 123FC for taking on board my security comments.