Guide pressure 21 bar => Limp mode

[Peugeot driver]

Hi,
I have a Peugeot 307, year model 2006, with automatic gearbox AL4. I get the very common Gearbox faulty error after driving about 5 minutes, only when starting with cold engine. The gearbox then changes to limp mode (3rd gear).
If I stop the engine and start again, the problem is solved until next time.

I have made diagnostics, I get error code P0006 (Pressure regulation fault). I have also logged data during driving. What happens is that the guide pressure suddenly goes up to 21 bar and the oil pressure to 11.25 bar. This leads to limp mode and the warning shows in the display.
The oil pressure sensor seems to work, the voltage changes with the pressure.
Is there anyone who recognizes this and knows the root cause? I have read that solenoid valve is one of the most common reasons. But I have also read that many have changed soleniods but problem has not been solved.