I am on Build 11.4 (11040) 

I want to use Active Directory Users to Login to the FrontEnd. Settings -> Admin Settings -> External Authentication -> Active Directory -> Import Users

My Domain Name, my Domain Controllers and my Username / Password are definetely right, but I get this message:

Error occured while enumerating Oraganizational Units.
Reason: Incorrect login credentials (or) DomainController is not reachable.

Where do I start troubleshooting this?