Symptoms

When clicked on K2 For SharePoint icon in SharePoint 2013, the contents not loaded with error "Content was blocked because it was not signed by a valid security certificate"
 

Diagnoses

Upon checking, the environment was configured to use HTTP (SharePoint) HTTP (Smartforms). The same certificate error was found when we try to browse runtime site with HTTP. This is due to Window STS Redirection which is using HTTPS by default.
 

Resolution

Modify the STS URL in Identity.Claimissuer table. This also can be done at Security Management page in Smartforms (http://help.k2.com/onlinehelp/k2smartforms/userguide/1.0.6/default.htm_authentication management settings/k2 smartforms authentication management settings.htm)

Blackpearl service restart is required here.