ok, it got harder to pinpoint in the latest update, as the lines touched seem to be way more, but i managed to fix it by simply deleting lines 248 and 249 in renpy/substitutions.py, as those call in the exception. if you don't have microsoft visual studio (it's free) or something similar to open the .py file, send substitutions.py here, and i'll delete the lines and send it back.
(ideally, you should also paste the original substitutions.py somewhere, then go change the save file's broken name in the game, and replace the original file just in case, but it shouldn't be necessary)