Dear Carlo,
Thanks a lot for the login details to your site. I can confirm that this issue with the validation is a bug with the current version of CRED. It works with Types fields as I've tested but not with generic fields as what you are using. But this is now fixed with the development version of CRED and Types (CRED 1.3 beta and Types 1.6 beta). I just installed it in your development site. Then I have tested the CRED form and its now working.
Please confirm and let me know how it goes. Thanks 
Cheers,
Emerson