We are also having the same problem with Thank You messages. What we have discovered is that a newly created event AFTER we upgraded to 2.7.3c will display a Thank You message just fine. However all events that were created BEFORE the 2.7.3c update will not display a Thank You page after a completed registration. We too are in dire straits to have this resolved.
Danster - this has been resolved completely in the latest 2.7.5f package. There was an issue on a small number of sites when SSL was in place that the thanks message was not shown. People in this old thread have confirmed through tickets.
I am having the same issue with 2.7.6a - Not sure if this has any relation to the upgrade from 2.6.9 but since I made the switch and dropped the jEvents sync I've had issues with the confirmation/thank you message.
Users are getting confused, especially when they are hit with an error message.
I'm using Authorize.net for card processing and I have an SSL on the site as well.
building252 - This is almost positively a setup issue of some sort. Please submit a ticket with exact details... event examples... etc. Are you using global messages or custom per event?
You may have missed something in the migration to 2.7 from 2.6. Have you re-saved all of the events? This has absolutely no connection to syncing with JEvents or not.