Symptoms

When assigning workflow start permissions to a SharePoint group containing a specific user won't allow the user to actually start the workflow. However when you grant workflow start permissions directly to the user it works.
 

Diagnoses

This issue can be diagnosed when user groups that were previously allocated workflow permissions are no longer being assigned to the group added in the management page. Causing errors such as "User cannot start the workflow".
 

Resolution

Within the K2 Appit for SharePoint app page there should be a link "Synchronise Groups" if you access this link you should be able to synchronise the existing groups from SharePoint to K2. Once the groups are resynchronised with K2, Group permissions should then be resolved.