Validation of Application Sharing Server or Web Conferencing Server fails

When installing OCS 2007 R1 and R2 Enterprise edition server the validation of the Application Sharing Server or Web Conferencing Server fails with the error code

Received a failure HTTP response.: Unauthorized
[0xC3FC200D] One or more errors were detected

I have encountered this problem several times now and in my case this was a problem where IIS blocked local access when you use the fully qualified domain name (FQDN) or a custom host header to browse a local Web site that is hosted on a computer that is running IIS. You may receive an error message similar to the following: “HTTP 401.1 – Unauthorized: Logon Failed.” If you try the same URL from an other computer you should be able to access the site so it is not a critical problem.

If you want to fix this so that your validations don’t fail, configure IIS to allow enterprise pool FQDN for loopback:

Click Start, click Run, type regedit, and then click OK.
In Registry Editor, locate and then click the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
Right-click MSV1_0, point to New, and then click Multi-String Value.
Type BackConnectionHostNames, and then press ENTER.
Right-click BackConnectionHostNames, and then click Modify.
In the Value data box, type the FQDN of your enterprise pool, and then click OK.
Close Registry Editor, and then restart the IISAdmin service.

Resolution found over at Technet http://technet.microsoft.com/en-us/library/dd441168(office.13).aspx