You definitely want to establish her with a good local exotics vet at her age, since you may be seeing more of the vet in the next few years.
Another thing to watch for is whether she is having any trouble with her legs. If she is getting arthritis, she may be resisting the climb into the litter box due to some pain (it can be common to onset in rabbits) and you might need a litter box with lower sides. If that’s the problem, then the vet will need to give you a treatment for that as well since she will be diagnosed.
Spockie was on glucosamine when his back legs started to go, which helped him feel better among other meds. But he couldn’t make it into the litter pan at all and I bought special rug remnants (Palace Pet Beds online) that absorbed pee and were washable. He clearly wanted to use the litter box, he just couldn’t make it. Incontinence is a sign of serious issues, but when treated well it can be disabling but not fatal. A disabled bunny can be a joy for years, so this is the worst case scenario, but there’s lots of help if it’s not a condition that will go away. In your case, it probably is a much more treatable issue and will go away for now. It can be as simple as the litter box location displeases her.