I am very annoyed as I have encountered the same problem.

I have exactly the same problem as the OTC. As Sassan tries to summon the demon the first words of the summoning comes up and then the game is stuck waiting for her to complete the summoning and eventually the game freezes and requires a reset of the console.

I have reloaded 3 times and every time it hangs I even loaded a couple of previous saves and worked again to this point but still the same problem. My character is level 21 and I really don't want to start over again.

I purchased the original Divinity II but struggled to play it due to the headache inducing poor framerate on the 360 however I enjoyed the game so bought the SDK version as hopefull everything was improved. So after £80 i still don't have a playable game.

I am playing the UK PAL Divinity II Dragon Knight Saga Xbox 360 version.
On a Halo LTD edition slim 250GB console the game is installed to the HD with over a 100 GB spare.