I have seen this error often the resolutions tend to be quite simple though either:
1) The IUSER anonymous account password is corrupt or has been reset.
Go to Active Directory, All Users, Right click IUSER_*SERVERNAME*
and reset password.
The go to IIS Manager, Right click the default web site, go to properties,
select Directory Security Tab, then click "Access and Authentication".
Replace the password with the new password you reset the account to then apply to all subcontainers when requested.
Restart IIS and this should resolve the issue.
2) The IUSER doesnt have permissions to view the Authentication module in OWA
Start IIS Manager.
Expand Server_Name, expand Web Sites, expand Default Web Site, and then expand Exchweb.
Right-click bin, and then click Properties. On the Virtual Directory tab, verify that the Read check box is checked.
Right-click auth, and then click Properties. On the Directory tab, verify that the Read check box is checked.