Same for me, wasted my time. The game is bugged in the middle of Episode five when climbing up inside the tunnel collapsing. It crash to desktop.

Quick search, found a way to go through. Gog launch CoJ, in the game folder there is a "CoJ_DX10" you can pass the tunnel with that (you just need to set the graphic options that are reset when launching this executable)

I couldn't get past the same exact section due to crashes, but I just want to confirm that the solution mentioned above works like a charm and it fixed the issue (starting the game with the DX10 executable)