I found out that if I comment out this part:
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upleft_coords.x = upper_left.pos_x;
upleft_coords.y = upper_left.pos_y;
upleft_coords.z = camera_height;
vec_for_screen(upleft_coords, camera);

upright_coords.x = upper_right.pos_x;
upright_coords.y = upper_right.pos_y;
upright_coords.z = camera_height;
vec_for_screen(upright_coords, camera);

downleft_coords.x = lower_left.pos_x;
downleft_coords.y = lower_left.pos_y;
downleft_coords.z = camera_height;
vec_for_screen (downleft_coords, camera);

downright_coords.x = lower_right.pos_x;
downright_coords.y = lower_right.pos_y;
downright_coords.z = camera_height;
vec_for_screen (downright_coords, camera);
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then the engine stops working, but if I comment out everything else and don't comment that part, then the crash happens.

what can be wrong?

thanks!

- Muhsin Kaymak