I am edting a project that I worked on a few months ago. Before I try to render the video, all of the clips preview fine and look excellent. During render, the process reaches a certain clip and then stops, giving me an unknown reason for stoping. When I go back to preview the video afterwards, the clip that it stopped at, including similar clips (they are all the clips that used the blue screen effect) are completely red, not showing the clip at all. VP 8.0

What's going on? How can I fix this? It's urgent.