Background
Organizations will often deactivate a Project Server user that no longer requires access to the server by setting their Account Status to Inactive. However, if the user has the Windows Authentication option selected and the user's account is later deleted from Active Directory, you will be unable to save any changes to this resource's Custom Fields. Instead, you will recieve an error similar to the following: The resource could not be saved due to the following reasons: The NT account specified is invalid. Check the spelling of the user name, verify that a valid domain name was included, and check that a duplicate domain was not used.
Resolution
When deactivating the resource, change the User Authentication option to Forms Authentication. You will then be able to make changes to this resource's custom fields without recieving the error.