This might be something to do with the LDAP Root Directory you have entered. In the module documentation it states : This usually takes on the form "DC=<organization>DOMAIN,DC=<location>"
In my situation this did not work. I had to use ou=users,ou=xxx,dc=xxx,dc=xxx.dc=org, in other words I had to put in the actual location in AD where the user objects were located to get this to work. So... try changing the Root Directory to add further details and see if that helps.