When trying to login to the Web Interface using AD credentials a domain login screen repeatedly pops up and doesn't accept credentials. This issue may occur after upgrading to any version greater than HelpMaster v10 which uses the Microsoft Dot.Net Framework v3.5 SP1.
This is a security "enhancement" that Microsoft have put into place with .Net v3.5 SP1. The full nature of this issue and the resolution is outlined in our knowledge base article here...