Bipper preheating lamp stays on

Yes, constant illumination of the preheating lamp, accompanied by a stored error, strongly suggests a glow plug issue. Given your mileage (208541 km), it's plausible. Do you notice any rough idling or difficulty starting, especially when the engine is cold? An engine misfire could also appear. Have you checked the glow plugs themselves?

I haven't checked the glow plugs directly. Is this something I can do myself, or would it be better to take it to a professional? What could happen if I continue driving like this?

While you could technically check them yourself with the right tools and a multimeter, given the potential for damaging the glow plugs or the threads in the cylinder head, it's often best left to a professional, especially if you're not experienced. Driving with faulty glow plugs can lead to poor cold starts, increased emissions, and potentially damage to the diesel particulate filter over time. I'd suggest a visit to a workshop.

Thank you for your advice. I took it to the workshop, and indeed, it was a defective glow plug. The repair cost me 440€. I appreciate your help!