Ok, I found a solution of the problem without changing anything in the pedal itself. Generally you have to find a way how to drop off the current slowly while powering down. It could be a different DC-adapter with a larger coapacitor or just a 9V rechargable power block as in my case. After the power block dries out, the power drops slower then if you would plug out the cable of the adapter. The settigs are saved after that. That worked 100% for my PF.
You can even make that easier if you connect in PARALLEL the adapter and 9V block. But it’s on your own risk.