Symptoms

Workspace Error "object reference not set to an instance of object" when opening process instances or process versions.

 

Diagnoses

After upgrading the K2 server to 4.6.11 we suspected a problem in the FilterCriteriaBar.ascx control which is a common factor between these pages.
 

Resolution 

There is a control called FilterCriteriaBar.ascx copy the control from a working k2 4.6.11 to the problematic environment.

The control is located at C:Program Files (x86)K2 blackpearlWorkSpaceSitemanagementcontrols.

Replacing this control fixed the issue