The link provided is very useful and this would definitely solve the issues which i guess everyone in here is facing and hope next time no one should face an issue like the previous one.
Finally I could fix the problem by setting up an LDAP connection with fictitious data in the login field and then erasing these content in the table isys_ldap from the database _data.
I know there isn't a perfecto solution, but at the moment i have no choice.