That is a very strange custom scene script. I don't understand why he's basically replaced the whole of DataManager with his own module. He hasn't actually changed any of those methods in any way. I suspect you're going to have other issues, because his script initializes the new replacement module, but not DataManager, but OTHER scripts still use DataManager. I can't comprehend why he's done it that way, instead of just overriding or aliasing the original DataManager methods.
In fact, this is probably WHY your in-game saving is not working.
Here are 2 ways to get around the bug. One is easy, and leaves his scripts as they are, only modifying mine. This will let you run the game, but I suspect you'll still have the saving issue. The other modifies HIS scripts to reinstate the original DataManager and ditch his questionable replacement module. I'm not sure what the purpose is of moving the character in the create_new_game method - I had to comment that out in order to get the intro playing properly, but it didn't seem to have any effect after that. The second method MAY resolve both the crashing issue AND the saving issue. But there might be other issues it introduces (I don't know, because I can't follow his logic).
Anyway ...
method 1: replace the following two methods in my Override Save Location script with what's below:
#-------------------------------------------------------------------------- # * Determine Existence of Save File #-------------------------------------------------------------------------- def self.save_file_exists? create_app_paths if !@savePath !Dir.glob(@savePath + '/Save*.rvdata2').empty? end #-------------------------------------------------------------------------- # * Create Filename # index : File Index #-------------------------------------------------------------------------- def self.make_filename(index) create_app_paths if !@savePath sprintf(@savePath + "/Save%02d.rvdata2", index + 1) endmethod 2: in the Custom Scene script, replace anywhere you see RPGDM with DataManager (one in self.run, one in self.run_map and the module RPGDM). And as I said, I had to comment out the DataManager.setup_new_game method at the bottom of the Custom Start script, because with it in, it went straight to the specified map and didn't play the opening cinematics.