Problems with authentication with Notification Server which may be caused by 'bad' aspnet settings which impact IIS.
Notification Server 6.0
IIS 5.5, IIS 6.0
IIS settings may be incorrect.
Resetting ASP.Net registration for IIS
Note: resetting the asp net settings is quick and won't adversely affect your Notification Server. It often helps correct authentication problems in the Notification Server.