Well it is half solved i guess.
After the payment, the client is transfered to our site where he still gets the message "contact the Webmaster. The basket could not be found".
But this the only problem because even when the error message shows the profile is updated with the right paid subscription.
Example for the same payment
Event type Change log
Event time 16/09/2016 23:47:35
Message Updating payment basket status: Completed because of event received: web_accept. Previous status was: NotInitiated
and the next log for the same order
Event type Error log
Event time 16/09/2016 23:47:50
Message ccbill: The OrderId is missing in the return URL: Probably due to missing "cbsubs_ccbill_basket" cookie: Please contact the webmaster to check the error logs.
Now if you look at the two logs recorded time you will see that the payment basket status changed 15 seconds before the error log was recorded. I do not know if this is somehow related to the "The basket could not be found" error.
Is there a case that the basket could not be found because it was already updated?