This is the bug where the player's one-handed stat inexplicably drops to 15 and has a red penalty marker indicating a disease or active effect, despite no disease being present.

I've already read a couple of reports of this happening online, and the general consensus seems to be that levelling up once tends to fix it. However, in my case, I've just reached 100 with no backup saves pre-100 so that isn't a viable option for me. Am I screwed? I worked for a good 50-60 hours to max that stat out over the course of the game and now my one-handed attacks are dealing pathetic damage. The only thing I can think of is finding a one-handed skill book I haven't read and see if that does anything.