Key Management Service event ID 5069

I am testing Key Management Services on our Exchange 2000 server. I enrolled members of our department. Almost all users have been able to enroll successfully, but a handful got messages back from the System Attendant saying “Unable to create a Security Setting. Please request a new Security Token and try again.”

I checked the Exchange server and in the application log there’s event ID 5069 which says the mailbox is not a correct state.

Can’t find any information on TechNet or Eventid.net.