DS4 Automatic Transmission Parking Issue

Those symptoms definitely suggest a transmission problem. While the parking mechanism itself could be the cause, the error messages and shifting issues point to an electronic control malfunction. Has the car had any recent transmission maintenance or impacts?

No recent transmission maintenance or impacts. Given that, is a defective control unit still the most likely cause, or could there be other explanations?

Without recent maintenance or impacts, a defective control unit is still highly probable. The transmission system is interconnected, so electrical faults often seem like mechanical issues. But, a real diagnosis needs professional tools. See a qualified mechanic to confirm the problem and prevent more damage.

Thanks, I'll get it inspected. The estimated repair cost is about 1000€, which sounds like a control unit replacement.