There are three known causes of this error:

1. When an email is sent from an offline device, CRM does a real-time check of the status of the document in Vault. The document has to be unexpired in Vault. Or users need to sync to update to the new version of the Email Template. 
   
   
2. In the Vault setup, the error occurs whenever Steady State is defined as anything other than Approved. Typically, Approved is the steady-state. The Multichannel (MC) Engine does not allow the sending of an email if the template, fragment, or any of the associated documents are not in a steady-state.
   
   
3. The field Allowed_Document_IDs_vod__c lists Vault Document IDs that can be selected for use within this template. The documents listed in Allowed_Document_IDs_vod__c must be set to Approved in Vault.

Related Documentation:

CRM Help Documentation: Managing Approved Email Content