Thanks! I found, that for me, I just need to connect the phones and then reboot the device (not the host). The battery levels had no impact.This was the method I found to choose the host device last time I checked it out: https://forum.xda-developers.com/t/...ices-platforms.2239421/page-145#post-84015381
I did, however, work out that I needed to start the ADB server as root otherwise it can't write it's log to /dev/tmp. I used
su -c adb start-server. The rest of the time I just used the normal user to run the ADB commands.