Which model are you using? Inside of LM Studio, you can see inside of the Developer Logs what the output was. There will be a lot there that can't be changed, but what you should look out for is the "content". That is what the game receives. Since you are getting an incorrect JSON formatting, try and see if the AI is adding anything that it shouldn't, or maybe leaving something out. Newline "\n" and space characters sometimes mess with the formatting, which can make it unreadable for the game. Maybe try replacing the prompt in-game to this instead:
Your entire response must be in json format with no line breaks or indentations (include choices in the json, not the game_text):