Is it so stuck that a good old scrub and soak of vinegar won't clean it? Musti loved to spill his dandrlion milk everywhere sk his bowl and blankies and everything were covered in them. These stains had stuck around for months but a good scrub of vinegsr and water cleaned it right up!
Also, a counter-question: i once sticky glued a pee pad to the plastic carrier by the pad's glue corners and Storm ripped the pad out, leaving the sticky corners in and they won't come off no matter what i try!
There are adhesive remover sprays that work well, acetone does too when soaked and covered with a plastic bag to prevent evaporation for some minutes, quite a lot of solvents like petrol or 2-propanol would, but since it's in a bunny area I would try simple vegetable oil and rubbing it off. Let it soak for some time.
Plain vinegar will work like nothing else. The trick is to use it full strength and let it soak for 30 minutes. It will break down the crusty urine buildup. You'll need a plastic scrub brush, but the vinegar will do the work.