Symptoms

SharePoint 2013 Event receivers not working, workflows not triggered on item added and other start rules tied to SharePoint lists.
 

Diagnoses

This is typically either caused by a lack of start permissions (which will be apparent in the error logged in the K2 Hostserver logs), or some other issue communicating with the event receiver.

K2 uses the SharePoint event receiver in order to trigger a web service call to the SP15EventService. If this call fails for any reason, the workflow will not be triggered.
 

Resolution

After enabling verbose ULS logging on the sharepoint server, It was determined that a certificate error between K2 and the SharePoint farm existed. Error Message:

Calling remote event receiver failed. URL = https://k2.denallix.com/SP15EventService/RemoteEventService.svc] ...
An operation failed because the following certificate has validation errors: Subject Name: CN=k2.denallix.com...

After ensuring the validity of the SSL certificate in use for the K2 workspace site, which houses the SP15EventService, we attempted to navigate to the service endpoint (https://k2.denallix.com/SP15EventService/RemoteEventService.svc) from all SharePoint servers. In this case, several of the servers did not have the trusted root certificate installed that ensured the chain of trust for the SSL certificate being used. After installing the root certificate on all SharePoint machines, workflows were able to start normally.