When you say default domain admin are you referring to admin@system-domain? If so, this is not part of your AD but is a local SSO user.
If this is some other user, web client seems to throw this error out randomly. I have no idea why.. but this workaround seems to work for some users:
See the post by dlund:
http://communities.vmware.com/thread/421398?start=0&tstart=0
Regards
Girish