Symptoms

Unable to initiate SP 2013 events-based workflows from K2Corp site. The site has been configured with the latest available K2 for SharePoint app and is pointing to a Azure based (Denallix Core 6.1).
 

Diagnoses

When changing the start-workflow option to manually start the workflow, the workflow starts successfully from K2 side. This appears to be related to certificate and port requirements. See the following link: http://help.k2.com/KB001443 and view the Certificate and Port Requirements at the bottom of the article.
 

Resolution

The SharePoint online Event Receivers will only work with a purchased certificate, and the K2 environment URL/end point needs to be accessible by SharePoint Online. If a core VM is using then it won’t work generally, but standard Forms using content via the broker can work. Thus, it's possible to start the workflow using the K2 form submit/save rule.