Symptoms

Getting the following error while registering K2 in one of the site collection.
“The underling connection was closed: Could not establish trust relation for the SSL/TLS secure channel.”

 

Diagnoses

The issue was caused by invalid cert for the SharePoint site. When browsing to the site itself, the user would receive certificate error.
 

Resolution

Recreating SP web app with the host name that will work with the SSL Certificate, redeploy K2 app, and was able to complete registration wizard successfully.