Try the following:
Start the course again and, instead of pressing the Orange Button that says "Start Training" or somethine like that on the top, go to the bottom and press the "old Windows 98" looking button.
Somehow that changed the color of the board for me (from greenish to brownish) and it allowed me to bypass the AJAX Error.
This might have nothing to do, but since that's how it started working form me I thought I'd share it. I became a Diamond Member only because it started working, not the other way around.
Best of luck pal, hope you are able to fix it!
Just subscribed to diamond subs in order to use Mentor; I'm getting same error ("An error occurred during Ajax response parsing") when trying the first Mentor ROE023 King & Rook vs 2 disconnected pawns on a mac running latest versions of OSX, Safari, Firefox, Chrome etc. I've updated Java and Flash, allowed pop-up windows and restarted the mac/cleared all cookies and temp files. Nothing works.
I'm on a broadband connection with no downtime issues. What can I try next to solve this please?