Electronic Parking Brake Failure

Sounds like a potential issue with the EPB system itself. Have you had any brake work done recently, or noticed any unusual noises coming from the rear brakes? The error code might give you a more specific indication.

No recent brake work, and no strange noises. Any idea how I can read the error code myself? Or do I need a special tool?

You'll need an OBD II scanner to read the error code. Many auto parts stores will scan it for free, or you can buy your own. However, based on your description, especially the lack of function and stored error, it's likely the electric motor within the brake caliper is defective. This is a fairly common problem. I recommend professional brake diagnostics.

Thanks for the advice. I took it to a workshop and they confirmed it. The electric motor in the brake caliper was indeed faulty. Repair cost me 505€.