Mercedes X Klasse brake light errors

Sounds like a faulty brake light switch. Check the switch itself first. Use a multimeter to check for continuity when the brake pedal is pressed. The error message and tail light malfunction confirm this. Start there.

Where exactly is the brake light switch located and how difficult is the switch replacement? Is there anything special to consider?

It's typically located near the brake pedal assembly. Replacement usually involves disconnecting the electrical connector and removing the switch, sometimes twisting it to unlock it. User error is a very common problem with this part. However, a workshop has the tools to do this correctly. So I recommend visiting a workshop.

Thanks for the advice. It turned out to be the brake light switch. A friend helped me replace it. The error is gone now and the brake lights are working again. Total cost: 0€.