W1508 means: You load a savegame that has nothing to do with your current script. for example: from other games I think that is not true

E1515: I think you have a problem with a null pointer. Maybe you have changed anything between save_game and load_game.

Try to save your game. Then kill the current level and load an empty level with level_load(""); And then try to load the game. Normally it should work. Otherwise there is anywhere an error.