Error adding domain user to local group

I am trying to add a domain user to my local Windows 2000 workstation and receive the error Processing of Object “username” failed with the following error: “The specified domain either does not exist or could not be contacted”.
NOTES: the workstation has a valid domain account, I am logged into the network as a domain admin account that has full rights to this machine. I receive a similar error when trying to promote a server to a domain controller.