Error 401.2 with OWA

Folks, I need some help. I've got an Exchange 5.5 SP4 server running OWA and authenticating via NT Challenge/Response.

Sometimes, and it seems to be intermittent, my users going to the site are greeted by: 401.2 Unauthorized: Logon Failed due to Server Configuration.

The error appears immediately upon entering the URL into the browser. No logon box is displayed.

This is the same type of error you get if, when presented with the logon box, you enter incomplete or incorrect information.
Seems to me that in certain instances, the browser might be automatically passing some type of authentication to the server which is then rejected, but I cannot find anything on TechNet that speaks to this issue.

Has anyone encountered this issue before?

Thanks in advance,

Joe