DS3 engine control malfunction

The symptom you describe, coupled with the error message, does suggest a potential issue within the engine control system. However, it is prudent to first eliminate simpler explanations. Have you inspected the cable connections at the battery terminals for corrosion or looseness? A poor connection can manifest in a similar fashion.

Good call! I hadn't thought that the battery cable connections could be the cause. So are you saying that a bad connection directly stops the engine from even trying to start? Like, is it possible for a faulty connection to mimic a full-on ECU failure?

While a complete ECU failure cannot be ruled out remotely, it is statistically less probable than a connection issue, which can indeed present in a manner that superficially resembles an ECU fault. The absence of adequate power supply inhibits the ECU's ability to initiate the starting sequence, thus presenting equivalent symptoms. Given the non-start condition, professional diagnostic assessment at a competent workshop is advisable.

You were spot on! Cleaning the battery cable connections and tightening them up got it started right away! I took it to a shop anyway to be on the safe side. They confirmed it and charged 155€ for the work, including some battery terminal spray. Thanks for the advice!