Clear AD config file before validating credentials

I have a Freenas 11.2-U5, which is binded to an Active Directory Domain.

Under "Directory Services --> Active Directory", when I have configured it the first time I have used my own acocunt "MyUser" and the relative password. Everything is working right when using this account. 

Now I need to change this account to a services account "ServiceUser".

When I click on "Save" or on "Rebuild Cache" I obtain the following error:

{'desc': 'Invalid credentials', 'info': '80090308: LdapErr: DSID-0C09041C, comment: AcceptSecurityContext error, data 52e, v4563'}

This happen with any user except the my own user.

I have double checked that "wbinfo -u" returns all the users correctly.