Are you overwriting those manual saves or making new ones and deleting the old ones? If you're overwriting your manual saves, they will end up corrupt as well.
I typically save mine as the first three letters of the character name and whatever number it is, so something like Rag1, Ydw47, etc. until I declare my own kingdom, which is where I make a whole 'nother "PreKingdom" save and start the numbering over again.
That way, I can not only keep track of how long that campaign has been going, but I can also easily notice if any of them have disappeared or need to be deleted (to save space and theoretically keep load times down.)
