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