OK… I have found the switch. (It is not the switch labeled BOOTCFG.) Set the switch to OFF and put everything back together.
No problem with the heatsink. I am now in rescue mode. But the script reform-boot-config does not work. It always wants to mount and write the SD card. The SD card (/dev/mmcblk1p1) does not exists. The eMMC is /dev/mmcblk0p1.
Tried it manually.
echo nvme > /reform-boot-medium
But after a reboot it boots the rescue system again.
Do you (or anyone else) have another tip?
Thank you
Nils