Runtime Error when loading

I am having a reoccuring error where the engine crashes after trying to implement certain features, and then never allows me to open my level again because of a 'Runtime Error.' I had this problem before with a previous beta, but now in beta 4 when I tried to implement time of day sun settings the engine crashed, and I cannot access any of my levels in my custom project anymore. I can open a basic, default level in the 'Play game' option but when switched to world editor and told to open my problematic level, the engine crashes and displays the same message.

http://img.photobucket.com/albums/v416/mistmod/error.jpg

Any pointers? I have a ATI graphics card.