Access denied if user is logged in on another blog


My network configuration:

- Multisite Privacy
- Join My Multisite (Per site registration activated on all blogs)
- New User Approve (Moderate each subscription on a blog)

Let's say a user visits for the first time a blog: he must register on this one to have access. Registration completed, he can visit this blog.

Now, this user is logged in but can't register on another blog.

How could I fix this ? I want to keep "Manual" in Join My Multisite configuration (admin moderate subscriptions on their blog).

Best regards,