Directory Server and Administrator: account issue

Just to clarify: I've already setup many other installation with AD integration and admin account linked to AD account without any problems.
I'm facing this issue for the first time.

I've setup the directory server (AD) with one account: the account works without problems (I've tried to intentionally put a wrong password and the error message appears).

The problem is that when I try to create an administrator for that account, if I put that account and click on Test Account I got error message like the account does not exist.

I'm a little bit confused.