@chrisgr99: Thanks for your solution. It works also on a Particle Xenon (nrf52840).
@all: Has anyone tried to completely remove the bootloader as they did for an Adafruit Feather M0 in the following post? Problems starting debug session with jlink on feather M0 - #11 by prototypicalpro
This is much faster than having to restart the debug session every times.