Workflow Restart after error not available in 4.6.c

[Robin Sahasranam]

We are in the process of upgrading form 4.0b to 4.6c. In the 4.6c upgraded test
system, I have several workflows in error because a workitem could not be
created because no actual agent could be determined (error WL 840  Agents of
type A cannot be defined). I have fixed the agent assignment problem. When I try
to restart the workflows in error, the Restart after Error menu option for the
workflow is unavailable (greyed out). I do this by using the Technical Work Item
Display, then Change work item.
 
In the 4.0 system, when I occasionally had the same error, I was able to Restart
after Error and everything was fine.
 
Can you please suggest why the Restart after Error functionality is disabled in
4.6?  It can't be authorization-related as I have SAP_ALL and SAP_NEW
authorization profiles.
 
Thanks for any assistance you can offer.
 
Robin Sahasranam
World Bank