After correct boot I get 'PPK command failed' when pressing 'START' or 'STOP'


I think having read and tested a lot of discussed topics here, but still I can not make run the PPK on the DK52 board (PCA10040).

I have prepared the HW by cutting SB9, placing both switches to 'DK' at the PPK and set the COM switch to 'DK'.

When starting the application and connecting the DK52 with the installed PPK I see messages like 'Validating firmware for device with s/n ....', 'PPK closed', 'Opening device with s/n ...', 'PPK opened', 'Initializing the PPK', 'Segger serial: 68....', 'SEGGER speed: 2000 kHz', 'SEGGER version: J-Link OB-SAM3U128-V2-NordicSemi compiled Mar 17 2020 14:43:00' and 'PPK started'.

But when I press then the Start-Button, I get a 'PPK command failed' message and an 'Average started' message in the log, but nothing to see at the screen...

I saw that for ony topic in the forum the nRF Connect SW V2.70 helped, but not in my case.

What can I do next?

Best regards,