Restarting Workflow with errors

[Robin Sahasranam]

We occasionally get role resolution errors in some of our custom workflow
template /customer tasks. After fixing the cause of the role resolution error, I
try to restart the workflow from SWI8 but get the error WI 726 'Restart of work
item &1 not possible'. What do I need to do to allow the workflow to be
restartable after errors? We use custom function modules for the role
resolution.
 
Grateful of any help you can offer.
 
Robin Sahasranam