Community Builder and Virtuemart problem

[The big]

Hello !!

I know this question has been asked a lot of times but i can't find any solution (up to date)

I have a problem 
I am using joomla 1.5.26 , Virtuemart 1.1.9 and CB 1.8.

When a client is at the checkout page, they get this message:
“Info: You are not a Registered Customer yet. Please provide your Billing Information.”
Even though they have already registered using CB.

Why is this happening? What has to be done?

Thank you for your help