I received some reports that my Max object ajm.ruby is not working in Max 5.1.2. I can verify this is the case. I am trying to get to the bottom of it, but my plate is pretty full right now so it might take a little while.
If you really need to run Ruby scripts in Max, I suggest you don't upgrade. Max 5.0.8 works fine and is available here: http://cycling74.com/downloads/older/