Well, after further trial and error, I can for sure say that it all boils down to (successfully...) starting the game in Japanese locale. Even with only the EN version of the RTP XP installed, it runs without errors when started in Japanese locale (which I do with Locale Emulator).johndoe1545
Perhaps a mention in the installation manual that you need the japanese RTP XP or a mention to change the RGSS104J.dll in the game.ini to RGSS104E.dll
I would even argue that changing the .dll file in the game.ini is counter-productive, because all it does is let people run the game in non-Japanese locale with text finally showing up and things (yaaay!) - but ultimately making the game unplayable with errors popping up (e.g. at orgasms/inseminations during battles, entering the slug cave near the tree after the glasses-girl joins, and probably the save/load error too, maybe).