I assume the service instance urls have also been updated to HTTPS?
Yes the service instance urls have been updated to HTTPS.
To further narrow down where the issue is, does it work outside of a workflow? Through the SMOTester tool, through K2 Management site, or a view?
Yes, the endpoint WSDL can be accessed via browser. I also run it via WCFTestClient and works perfectly well. The SMOTester also updates the services. But when the WF tries to invoke, it comes up with:
ServerEvent: Message: Could not establish trust relationship for the SSL/TLS secure channel with authority 'trackerdev2016:9191'.; ServiceName: MatterWCFService; ServiceGuid: 7b0c04c0-89cd-4383-80cc-754655cd9ece; InnerExceptionMessage: ;
InnerException: Message: Could not establish trust relationship for the SSL/TLS secure channel with authority 'trackerdev2016:9191'.; ServiceName: MatterWCFService; ServiceGuid: 7b0c04c0-89cd-4383-80cc-754655cd9ece