Finally I have been able to solve the riddle and I’d like to share the solution with you, although it has nothing to do with dietpi 
A couple of days ago I started testing again and hooked up my Small Form Factor PC, that I bought refurbished on ebay.
And as you can guess… the excessive DNS requests to “dwr.rijksoverheid.nl” where back again.
I did some research and tried to isolate the source of these requests.
The SSD that came with this PC has never been in use and there wasn’t even an operating system running on this machine, yet. So it had to be something else the PC was doing on its own 
I found out that the PC comes with Intel AMT functionality (a feature to control the device remotely) and this management technology is even running, when the PC is connected to mains, but powered off.
Another look through the BIOS and a reboot later I could access the settings of this feature.
And here we go…
Since the PC has been refurbished, it has obviously been in use for the dutch government and configured accordingly.
What a relieve, when I finally found out the root cause of this little mystery 