i'm not an expert, but does that appear all the time or only when you are loading a saved game?
try the following:
got to: %appdata%\RenPy\ExtracurricularActivities
back up all those saved files, (you might find it easier to zip them up and delete the saved and the persistent file)
also do the same in the game folder in the saved files, then
start a new game, if it worked, that means, the updated version of this game and/or the renpy version doesnt allow the old saved files..
for me i had that because i used the brackets in the name --> [ ]
hope it helps