In this example, I'm using the domain office and the username djcrumple and I'm using PRTG v13.
System Administation > System & Website > Active Directory Domain is set to office and I've created a user group in PRTG that links to one of our AD groups. I haven't been able to log into PRTG with windows creds, and I have tried logging in with and without the domain on the user name. I get two different errors in the log depending on how I log in. Errors are from PRTG Core Server.log
Error: 10/3/2013 4:15:47 PM Webserver: GroupEnum2: 80070035: The network path was not found
Error: 10/3/2013 4:11:47 PM Webserver: ADsLogin1(LDAP://office|office\djcrumple): 8007052E: The user name or password is incorrect
We've tried multiple times with different user accounts, so we know the passwords aren't actually incorrect.