The car is a VW Passat B6 2.0 TDI from 2008.
There is sometimes a flutter-like sound when going from about 3000 RPM down to idle (that is, by pressing the clutch). At over 100 km/h, depending of when the engine is pushed a bit more, it goes into limp mode (less than 100 km/h if it's doing a DPF regen).
The diagnostic says that is a turbo overboost condition. My best guess is that it is caused by a faulty wastegate/turbo actuator, but before having a shot at replacing it, I'm looking for a bit of advice, other things that could go wrong or other tests I could make to check what could be the issue.
One recent change was that the O2 sensors were changed - the flutter sound was present sometimes before (especially during regen), but not as pronounced and not going into limp mode.