Balazs,
There's a few different possible causes for this, but I won't know the specific answer until I know more about your configuration. What version of JFBConnect are you using? Do you have any of the profile plugins enabled? What's your site address? Sometimes, looking at the HTML on the page you're trying to login from will point out some common misconfiguration issues.
Finally, please post from the account you subscribed from. Priority support is for subscribers only.
Thanks,
Alex