Exchange 5.5, OWA on Win2K server

Scenario:
Exchange 5.5 on Win 2K server using Outlook Web Access via IE 5.5. This works well with my Win98 clients. Using a Win2K professional client I can access the Logon screen of OWA but I cannot get logged into my account. The same user account can be signed on using the Win98 client.
What am I missing here??