Cannot get domain mapping to work

The website I was happy with I turned it succesfully to a multisite environment, we are talking about grootgroningen.com. Now I build my second website http://dekrantnieuws.grootgroningen.com and it is ready for release to the public.

I want http://www.dekrantnieuws.nl domain mapped to the website positioned on http://dekrantnieuws.grootgroningen.com.

I have installed the plugin wpmudev domain mapping, repositioned the sunrise.php and setup the plugin in the networkmanagement-area of my wordpress environment. I entered the ip-adres of my VPS. This VPS hosts about 10 websites, which are all mine.

I went to my domain-provider and changed the DNS (a-record)to the ip-adres of my VPS-server. The TTL of this domain was and is 4 hours. I changed the DNS 8 hours ago and when I ping to this domain, it shows the ip-adres of my VPS.

Next I go http://dekrantnieuws.grootgroningen.com/wp-admin and try to add the domain http://www.dekrantnieuws.nl into the domain mapping plugin and it gives me the following error: Domain name is unavailable to access I can read in the description that i can take up to 72 hours, but since the TTL is just 4 hours (the domain exists for a long time, so it is not new) it should work now after about 8 hours.

Am i to impatient?
Do I need a unique domain-name for every subsite of a multisite environment?
What am I missing, what do I need to double check?

I am using the latest version of wordpress 4.5.3
I am using the latest version of wpmudev domain mapping