What font are you using? Not all fonts have all the glyphs in; though I must admit that é would seem an odd one to be missing. I wonder if one of the font files you’re using has got corrupted somehow.
Actually, thinking of it more constructively, maybe the problem is with the font that you are using in your external editor — is it the same one as you use in Scrivener? — that that is the one that has got corrupted.
Just random thoughts; I’ll leave it to those who know more, but I’d be looking at the fonts involved on your machine.