User role not updated after payment

Hi, I’m building  a membership site with WordPress 3.3.1, Buddypress 1.5.4 and S2Member free 120219 and I’m considering to go Pro in the future but first there’s a thing that must be resolved: I have two membership levels on my site, “Free Suscriber” and “Paid Member”. I have enabled “open registration” on the site. I have noticed that user role is not updated correctly after payment, because after payment the user appears as both “Free Suscriber” and “Paid Member” at the same time. The users cannot access protected content since they are still asigned the role of “Free Suscriber”. Here they are the two approaches tested and encouraged to create a membership on my site, the first works and the other fails:

Case A:

User pays > S2Member shows page saying: “Thanks your account has been approved” AND shows “register” button, but this button is ignored > User clicks on email link > User registers account > User receives activation link on email > User activates account and logs in the site > User appears in S2Member member management panel as “Paid Member” > User can access protected content = Success!

Case B:

User pays >  S2Member shows page saying: “Thanks your account has been approved” AND shows “register” button, this time User clicks on this button > User registers > User receives activation link > User activates account > (this is where the bizarre thing starts) User appears in S2Member member management panel as BOTH “Free suscriber” and “Paid Member” > User cannot access protected content = Fail

Why the users in case B are not updated as “Paid Member” and their “Free Suscriber” state is not nullified? What I’m doing wrong? 

Lots of thanks in advance for the one who can clarify me the cause of this behavior.