Skoda Superb diesel engine problem

Reduced engine power, smoke, rough running, increased fuel consumption, jerking, hissing... sounds suspiciously like a vacuum leak in the EGR system. I had a similar problem with my Superb. The check engine light is a dead giveaway. Have you tried checking for any obvious signs of a leak? Sometimes you can hear it, especially when accelerating. What sort of driving do you usually do, short trips or longer runs?

Vacuum leak? How exactly do I check for that myself? What does that even mean? Also, why would short trips matter?

Checking for a vacuum leak yourself can be tricky without proper tools. Basically, the EGR valve is part of the exhaust gas recirculation system, which reduces emissions. A vacuum leak means unmetered air is entering the engine, messing with the air-fuel mixture. This can cause all the symptoms you described. Short trips are bad for EGR valves because they don't get hot enough to burn off deposits, which can cause them to jam. Given all the symptoms, it's likely the EGR valve itself is faulty. I'd recommend getting it properly diagnosed by a workshop before attempting any repairs. They can confirm if it's the EGR valve or something else causing the check engine light to come on.

Took it to the shop as you suggested. It was indeed the EGR valve. Replaced it for 355€. Runs like new now. Thanks for the help!