Seat Formentor Diesel Engine Trouble

Sounds like a faulty intake manifold pressure sensor to me. Had a similar issue with my old Audi A4 (different engine, though). Check engine light, sluggish response, the whole deal. The manifold sensor, sometimes also called a pressure sensor, is an automotive sensor that measures the pressure in your engine's intake manifold. A large vacuum leak can definitely throw off its readings and cause all sorts of problems. Have you checked for any obvious cracks or loose hoses around the intake manifold?

You mentioned checking for cracks or loose hoses. Where exactly should I be looking around the intake manifold for potential vacuum leaks?

On the Audi, the most common spots were where the hoses connect to the intake manifold itself and around any vacuum lines running to other components. Given the symptoms and your suspicion of a vacuum leak affecting the manifold pressure, it is probably the sensor giving false readings. I'd advise getting it properly diagnosed at a workshop, just to be sure.

Replaced the manifold pressure sensor. The vacuum leak was indeed the problem. The total cost was 235€, including the sensor and labor. It runs like new now, thanks for your insights.