Had this problem for 3 weeks, exactly as identified in the first post.
Fixed this by :
Computer mnagement,services and applications,internet information services,web sites.
Right click on default web site and click on properties. Click on Directory Security tab. Click Edit in the authentication and access control. Select enable anon access and change the account to the administrator account. Type in password as appropriate. Click OK on bottom the apply. Then go back into the same section again and take off the tick to allow anon access. When you click ok and apply you will get a two boxes one after the other which will ask you to apply settings to the objects in the default web site. Select all to apply in both cases.
Try OWA now.
The only other thing is that I applied the microsoft fix mentioned in an earlier post.
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