XC90 brake lights failing

I have a Volvo XC90 too, and I remember reading something similar. It's often just a bad connection or a faulty switch. Have you checked the brake switch itself? Maybe try cleaning the contacts before replacing it.

Where exactly is the brake switch located, and is it easy to access? I'm not very experienced with car repairs, but I'm willing to try before going to a shop.

It's usually located near the brake pedal assembly, under the dashboard. You should be able to see it if you get down there. But honestly, if you're not comfortable, I'd recommend a workshop. Messing with the brake system could cause safety issues or user error.

I took it to a trusted mechanic. It was a simple issue with the switch connection, likely from me kicking it while getting in. They re-seated the connector, and the error is gone. Repair cost was 0€!