The nameservers you declared are more than likely misconfigured:
ns1.trdns.net, 213.238.128.125
ns2.trdns.net, 213.238.128.225
The following error(s) occured:
errors=2, warnings=0, informational=0
* domein assortie.nl.
* nameserver ns1.trdns.net./213.238.128.125 E server onbereikbaar of timeout (status: 'query timed out')
* nameserver ns2.trdns.net./213.238.128.225 E server is niet 'authoritative' voor assortie.nl.
Please correct this as soon as possible and keep us informed by mail on staff@eurodns.com so that we can proceed to the registration of your domain.
With many of the ccTLD registries, the domain has to be set up and active before it is included in the zone. The .de registry has checked ns1.trdns.net and the query timed out. It then checked the second dns and got no answer.
ns1.trdns.net seems to be down/unreachable. Or the nameserver program is not running on that nameserver.
Regards...jmcc