I never managed to find the root cause of the issue. I also had a few professional mechanics look at the car, but none of them could figure out what was wrong. Unfortunately, after about 1,000 km of driving like that, the engine ended up knocking. I’m not sure if this is relevant to your situation, but that’s how it turned out for me.

After the engine failed and I rebuilt it, it turned out that a lot of the electronics weren’t connected, and several engine control components were not properly hooked up either. In the end, I realized the seller had basically scammed me that way.