Hi,
That's because after the registration process, HikaShop tries to auto log you in, since it knows your username and password at that point.
But for some reason, this fails.
I don't know why this would fail as it works fine on our end. I did the same test on your website and on mine with the same version of HikaShop and it worked fine on my end.
I'm afraid we'll need a backend and FTP access in order to debug the issue.
Could you provide that along with a link to this thread via our contact form so that we can analyze the situation ?
www.hikashop.com/support/contact-us.html