Did you recently upgrade vCenter? Reinstall?
I've seen this before when the "Customization Specifications" are "upgraded" and the Administrator (and/or) a Domain Account password isn't reentered into the Customization Specifications.
Have you reset the password in the Customization Specification? Or possibly created a new Customization Specification?
Jase McCarty
http://www.jasemccarty.com
Co-Author: VMware ESX Essentials in the Virtual Data Center (ISBN:1420070274) Auerbach
Co-Author: VMware vSphere 4 Administration Instant Reference (ISBN:0470520728) Sybex
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