Fixing the Housebuilder sample

If you are interested in Silverlight 5 and its new 3D graphics capabilities, you probably know about the available samples on MSDN. One of them is the Housebuilder sample John Papa first presented during MIX11. For quite some time I've been struggling with this particular sample, because all I got when I tried to run it on various computers was this:

At first I thought it was a bug with Silverlight 5 (it is a beta version after all), but the setups of the computers that showed the problem were so different in both hardware and software that is seemed unlikely. Another computer that was almost identical to one that showed the bug ran the sample without problems. What was the problem? More...