Symptoms

We can't open Smart Forms from SharePoint K2 viewer webpart from IE 11 , such that it gives "Content was blocked because it was not signed by a valid security certificate" , i added DNS on intranet zone no luck also i tried to add it to trusted zone same error , please your urgent support is highly appreciated , we are planning to go live next monday .
 

Diagnoses

Use Https for Sharepoint and SF
 

Resolution

It's always highly recommended to use both HTTPS for both Sharepoint and Smartforms site. We only support these two scenarios: 

1) Sharepoint (HTTPS) Smartforms (HTTPS)
2) Sharepoint (HTTP) Smartforms (HTTP) - Not recommended.

Read Supported Schema Configurations at http://help.k2.com/onlinehelp/k2forsharepoint/icg/1.0/default.htm

If you are in scenario 2, there is an additional steps:-
refer to "Workaround for configuring a K2 for SharePoint On-Premises for non-SSL based connections"