I have read and re-read the postings about this issue, and I have come to the conclusion that it just isn't working.
I am on 2.7.3a - just have not upgraded one more time but will do it...
I get this message whenever setting up an event from the front-end. It will work if done from the back end.
The various things noted all seem to be in place... and as I said, the event registrar gets the message if the event is set up from the backend. But my users want to do it from the front end.
There is ONE place where this remains... email notifications after frontend event management are not working correctly yet. So for now, uncheck the boxes in configuration that say to send emails for these actions. You will not get this error message on any other function such as actually registering for events. This frontend management notification will be fixed soon.