No redirect after choosing subscription

Hi guys,

First of all, I'm very sorry to ask this question. I am quite sure I'm doing something really stupid and the answer is in the line of 'read the manual'. But I've been spending two days on this, and just can't seem to figure out how to make it work.

The problem is this:
I've got a multi network with users, and a specific site with membership activated. Current users can choose a membership subscription from the site on which they already have a user account.
The problem is that, when they are logged in and want to choose a subscription, the are not redirected to the next page.
More specific: after clicking the 'sign up' button, the URL changes to /subscriptions/?action=registeruser&subscription=2
I think (not sure) the membership plugin is redirecting to a register page, but the user already is registered (and logged in). As a result, the signup options (subscription page) is shown again.
Any idea how I can fix this?

To be more specific: Is is online on http://bimprotocolgenerator.com/subscriptions/?action=registeruser&subscription=2 Feel free to try it and test it with by creating an account.