Logout When add new Address

I dont know why its happening this:

When i'm on the checkout, ready for place my order and i login with my admin password it retrieves me that i dont have any shipping address.
Then i click on the "new" button and it opens the popup displaying the "add new address info"
At the end, when i fill the form and click ok it does'nt save any data and logouts...

I have no idea how to overcome this.

Can you give a look here ( hypershopping.altervista.org ) i have just one product by now for testing pourposes and it is this one

i've tryed changing the template but the problem persists.

Is so frustrating....

(joomla 2.5 + HS 1.5.6)

Thanks in advance

Ya, i'm having a lot of troubles in the last week with that website, i don't know what's happening in there with the registration stuff and i'm willing to begin everithing from scratch.
Thanks for your time, i hope that with a new fresh installation everithing goes better.
Have a good day