I have the same problem; when a user clicks a game (Millionaire) in a lesson, the user is forwarded to the main page of the website.
Our Moodle version is 2.9.2. The website is almost a year old. Everything else works perfectly. A day ago, there was no problem when we created the game. Now we have the problem mentioned above. Debugging is on but there is no message displayed. I tested this on Firefox and Chrome, purged all the cache, changed the lesson format from Tab (there was a debug message) to Topic format, still no solution.