This is a VX scripting problem.
I'm not exactly sure what's causing this, and I can't really seem to consistently recreate it, but occasionally when I try to save my game while test playing, I get that error and the game crashes. Not only does it crash, however, but it also deletes my save file that I was trying to save onto.
Possible conflicting scripts that I have are Dargor's Custom Commands, Large Party, and Advanced Files However, I'm under the impression that it's not the latter script that's causing a problem because I got this error once before I plugged that script in.
I don't know if it would be the Large Party's fault either, as I only had 4 party members at the time, which shouldn't cause any problems.
Any ideas? :\
This is the line in scene_file for reference:
Marshal.dump($game_system, file)