There are a number of things that could be causing this. However, to understand things better, we would need some logs or indication of some error. Depending on what you are using as your AD server, it must have logs to see if the server is rejecting the access request and why. You mentioned that you tried the "Test Authentication/Accounting Server Settings". However, you did not mentioned if when you tried that, it was successful or not. The system will typically give you an error if it was not successful. Did you see that?