viking3r

So the prior replacement of the linux-image seems to be the culprit
 
Makes sense in hindsight
 
I now installed wireguard via 
 
sudo apt install -t buster-backports wireguard wireguard-dkms wireguard-tools linux-headers-legacy-odroidxu4  
now it works
 
I rebooted a couple of times no issues there. 
 
So in the end it seems like it was the install of wireguard who led to all of this. 
 
The one time I could reproduce the device not rebooting without having it installed must be something like tparsys said. 
 
 
So for now the problem is solved. And some knowledge was gained
 
I should pay more attention when apt wants to install some packages.