There was an error in communicating with Excel Calculation Services

Tried to install MOSS 2007 and integrate Primavera With MOSS 2007. However, I got the error message below when testing Sharepoint Server 2007 with Excel Services:

There was an error in communicating with Excel Calculation Services http://sharepoint2007:56737/SharedServices1/ExcelCalculationServer/ExcelService.asmx exception: The request failed with HTTP status 401: Unauthorized. [Session: (null) User: MyLocalDomain\administrator].

I tried all recommendations I got from Internet about this error in the Sharepoint 2007, including using Kerberos authentication, but none of them works. However, I got a tip from an article from Microsoft about using authdiag.msi, I installed authdiag.msi and it detected the error below:

AllowKeepAlive: NTLM requires KeepAlive connections

Then I jumped to IIS Manager, and checked Enable AllowKeepAlive option for all Sharepoint Websites. Immediately, my Sharepoint 2007 was able to open an Excel Workbook in the report dashboard, and the annoying error of "The request failed with HTTP status 401: Unauthorized. " had just gone. My life was saved.