Bug following sign up for another level of membership without completing payment

I have different level of memberships : http://www.culturesenior.org/register/
One of them is a free 24h premium membership : I have noticed that when I choose this free membership and then decide to click "sign up" on another level of membership WITHOUT completing payment (http://i.imgur.com/89pHZMZ.png), it creates a bug and even though my free membership would normally allow me to go past the "protected content" I hit it anyway... Then I'm stuck.
Thanks for your help!
Thomas

(I have granted access to your support team)