@tshering @NiLuJe: That's what I should have done in the first place
I installed an unpatched firmware, and - voilą! - my missing letter is back again. After installing KoReader still no missing "O", but after applying the patches my keyboards lacks the "O" again.
So sorry I suspected it might be KoReader's fault! Next step is to find the responsible patch, but not today
Thank you all for help and suggestions!