Problem with an existing user who wants to create another subsite

Hi,

When a user is logged in my main site, if he wants to create a subsite, he's getting redirected to the form that asks him to register on the site.
You can try it here:
http://creersonsite.net/creer-un-site-internet/
I'm using Gravity Forms User Registration integrated with New Blog Templates.

This problem occurs with an existing user who wants to create another subsite, or with a user who registered first without creating a subsite and wants to create one now, or with a user who wants to create several subsites...

How could I solve this problem? Any idea?

Thanks!
Best Regards