Can't even read past conversations that are deemed "too long": redirected to homepage with error "This conversation is too long, please start a new one."

Okay, that’s weird. Have you tried to delete some of your old chats/conversations and see if that would help? If that doesn’t help either, then in that case I would contact the support through Help, so they can take a look at your account. You can include everything you’ve already tried as screenshots.