This report came through recently: https://bugs.mojang.com/browse/MC-269934

Since mod issues aren't accepted at Mojang's bug tracker, having a message like this popping up in modmenu is sort of misleading. I don't know what would cause this to show up since it's never displayed when I use modmenu, but it might be prudent to put some kind of safeguard in place to stop this message appearing to prevent bug reports like the above from being made by misled players.