Error when adding user to membership


I'm using the Membership 2 Pro plugin, and everytime I add a Wordpress user to a membership I get a fatal error saying "You cannot register a new account. because you are already looged in".

I'v e tried it adding user that already exists as Wordpress users and adding users that didn't exist in the Wordpress installation. In both cases I get the same error.

Can you please help me to solve it?