Symptoms

This is Sharepoint 2013 running in Sharepoint 2010 mode. The 'Reports' in the K2 Process Portal (Activity Statistics, Process Instances, Process Instances, User Performance, Process Information) errors with 404 with the following message in the URL:

"Please enable Session State. Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive"
 

Diagnoses

"Reports accessed through K2 Process Portal are not supported. K2 Workspace needs to be used for reports."

http://help.k2.com/kb001380
 

Resolution

As this is not supported, some possible workaround may be:
_1
Add a link to K2 Workspace for viewing these reports. Access to some of the tabs in K2 Workspace can be restricted to users using the "Security" option:
http://help.k2.com/onlinehelp/k2blackpearl/icg/current/webframe.html_Post_Install_K2_Workspace_Security.html

_2
Create Smartforms that makes use of the Smartforms Report Controls and add a link to these Smartforms.
https://help.k2.com/onlinehelp/k2smartforms/controlpack/1.0/default.htm_Report Controls/Report_Controls.html?Highlight=report controls