Cats are quirky and can have many reasons to do that. If she is an older cat she may have arthritis and the walls of the box could be high for her to easily get over. Some cats hate covered litter boxes. If you changed litter brands recently she may not like the texture or smell. The box could be too small; I bought a large metal litterbox for mine before they quit peeing over the sides on the wall.
Emotional issues can cause outside box use too: new pet, roommate, someone moved out. All can have an effect.
Feliway has a pheromone spray or diffuser you can use that is supposed to help with issues like urinating outside the box which may help with the poop problem. Another brand called Comfort Zone has a "spray and scratch" that's slightly cheaper.
If the vinegar or peroxide that others recommended to clean the potty spot don't seem to do the trick, then you can buy an enzyme cleaner like Nonscents "stain and odor eliminator" to see if that will work.
I feed Instinct Ultimate Protein Chicken dry and Weruva Meowionaire wet food. Feeding only wet food with 3 cats is too expensive for my budget currently, so we do a mix.
One of the 3 has vomiting issues that were daily with wet food, and it was trial and error until I found a dry food that didn't make him puke every other day.
I have microchip food bowls so each cat gets the right amount. Vomit machine eats 1/4-1/2 cup of Instinct daily and a tablespoon of Weruva wet food. The other 2 each get 1/8 cup of Instinct per day and each get half a can of Weruva in the morning and evening. So I go through 2.5 cans of Weruva a day and 1/2-3/4 cup of Instinct.
Also, the Instinct Ultimate makes their fur really shiny. I noticed during a food shortage when I switched to Tikki Cat Herring/Chicken dry that their coats dulled, and 2 months later when I was able to get Instinct again they got shiny. The Tikki Cat did not cause vomit machine to puke, but I like the Instinct better.
They have 2 fountains that need refilled frequently, so I'm assuming their water intake is good.