test SCEP - error 503, service unavailable

Hello,

I completed SCEP NDES installation on a Server 2008 R2 VM following the procedure document on http://social.technet.microsoft.com/wiki/contents/articles/9063.network-device-enrollment-service-ndes-in-active-directory-certificate-services-ad-cs.aspx

Then I tried to test it by accessing http://localhost/certsrv/mscep_admin using IE 8, it shows error 503, service is unavailable. In the IIS Application Pools, I noticed SCEP gets stopped after some while.

Any step I missed in SCEP NDES configuration?

Application event ID: 2 and 8

Thank you in advance