mod_gk_register problem

I'm trying to enable captcha for my registration form that ,at the moment, is using a k2 User module type.
When I go to the registration page of www.assembleaapostolica.it with k2 captcha enabled (public and private keys already in) I see the image of captcha appearing and quickly disappearing.

Then I tryed to publish a module of gk_register type in the same position on the home page but, surprisingly, whe I do it the entire site is not showed.

Could you help?

Hi,
Now I see: "Plugin Captcha non impostato o non trovato. Contatta un amministratore del sito"
1) which version of K2 you're using? I suggest to use a new one, downloaded from http://getk2.org/ - just upgrade.
Try this, if not help send me a info + access to backend (login/password/URL) to priv.message I'll help!
Because it looks like JS conflict

2) I suggest to turn off all features of editor in Descrizione field (FrontEnd), who knows what people or spamer add there!