I have used LDAP module with Drupal 6 without issues, but now that I am working with Drupal 7 the LDAP module seems to be unable to load the AD groups. I have searched all along the web but I couldn't find any help.
The returned error on the watchdog simply states "unknown error: 6"

The complete watchdog row is:
ldap_authentication pippo.4 : Authentication result id=0 auth_result=6 (unknown error: 6)

The same authentication works fine using Drupal 6+LDAP.
What could be wrong?