You say you've got 5 cats, but maybe not enough litter boxes? I would definitely place, as SCM suggests, a box in the corner for a while plus an additional one upstairs somewhere else for a while until she's going downstairs more. I suggest you keep her confined to one room (with 1 or 2 of your other cats to keep her company) while you're at work all day and can't keep an eye on her. After cleaning the corner spot well, gradually move the box out of the corner toward the basement stairs (you might cover the corner with some aluminum foil in case there is still some odour there). Cats don't like to step on it. Do give her lots of praise when you do see her go in the box.
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