Ok, here's the deal. LTI Publisher is on Moodle 3.5, the LTI Consumer (me) was on Moodle 3.3.2 using Lambda Theme.
Setting up the course using LTI cartridge and secret was easy. Consumer key used on the Consumer side was my company name (Orion). Got green check marks, everything looked good. Saved the entry. Clicked on the course to launch it and got the following message:
An error occurred when launching the external tool:Sorry, there was an error connecting you to the application.
After much troubleshooting, retesting - including testing the cartridge and secret using the https://ltiapps.net/test/tc.php emulator - we determined that the cartridge and secret worked perfectly, so we decided to change both moodle versions to 'Clean' and retest the LTI links again. Still the same response received, as above.
The LTI publisher suggested that maybe we should upgrade to the latest version of Moodle, which is currently 3.6 Stable, so we did and retested the LTI cartridge again and we achieved the same result.
We are using LTI1, both the publisher and consumer are HTTPS, nothing seems wrong with the cartridge and secret so what's standing in our way?