After you logged in.
Go to yourdomaincom/index.php?action=registernew
Now you can register, while you are logged in.

How can I solve this? How can I change this behaviour?

When you login as a member (not admin), what do you see?

Probably, you have meant:

- Bug: you can't register while you are logged in!
- Now you can't register, while you are logged in.

?

Because in basic miniBB, there is not possible to register an account for the logged user. "Register" link should change to "Profile" and lead to the Profile.

"Registration" from the logged account possibly means you're logged in as the Admin.
But it's not a bug - it's a feature. Admin could create new users that way.

thanks for the help, I get it.

Admin can register while logged in, by going to domain/index.php?action=registernew

A normal uster can't register, while logged in.

Yes, admin has an option to register users manually (some forums need it).