Problem on registering new user

Hello , I set up EASYDISCUSS, but have serious problem on new registration ,

When a new user try registering he get this error :

Attenzione
Registrazione fallita: Registrazione fallita: Duplicate entry '0' for key 'PRIMARY' SQL=INSERT INTO `g6305_discuss_users` (`nickname`,`description`,`url`,`params`,`points`,`location`,`signature`) VALUES ('','','','{\"facebook\":\"\",\"twitter\":\"\",\"linkedin\":\"\",\"skype\":\"\"}','0','','')

Existing users have been migrated from a previous Joomla

Could you please help me ??