I had this problem too - and now have it solved. At first, I did as Swizzy said and unload the FSD plugin and the dash launcher. As soon as I did that and tried launching the game, it started trying to update my xbox! Of course, this did not work and it said it could not read the disc. My xbox is rgh and updates are disabled. The disc was straight out of the box so I knew there were no scratches. So - what I did to solve everything was install the newest kernel. Before you think it's too hard, it's really not at all. It honestly took 10 minutes using this guide. Remember to update your dashlaunch to 3.16. If you don't know how to do that just google it...but it is included in the links download:
http://www.se7ensins.com/forums/threads/how-to-update-jtag-rgh-nand-to-dash-17489-easiest-way-possible.1034192/#post-7780514
This will update your kernel to the newest (I think Nov-2015) and allow dimensions to work without any problems. At last, my kids can finally play their xmas gift!
