DNS is horribly broken. You say you checked nameservers and DNS settings -- I'd love to hear the description of what you found, because I'd call it an atrocious mess.
serifd.nl has nameservers
ns2.vlasveldmedia.nl. When querying those nameservers, I'm redirected to
ns2.nns365.com. Nevermind that such a redirect is already broken.
So, to find
ns2.nns365.com's IP address so I can ask them about
serifd.nl, I have to check the namservers of
nns365.com. But guess what, they're
ns2.nns365.com, the very machines I was trying to find in the first place. Boom.
Let's look at
turnbroekjemetnaam.nl. Well, the nameservers are
ns2.knothhosting05.nl. But guess what? They direct me to
ns2.vlasveldmedia.nl. And it's the same nightmare all over again.
Oh, and here's the really fun part. When I ask
ns2.nns365.com) what nameservers to use to resolve
serifd.nl, they tell me
126.96.36.199. And they respond authoritatively.
serifd.nl. 3600 IN NS ns2.nns365.com.
serifd.nl. 3600 IN NS ns1.nns365.com.
ns2.nns365.com. 3600 IN NS 188.8.131.52.
ns1.nns365.com. 3600 IN NS 184.108.40.206.
Great, they each tell me to ask the very same nameservers I just asked.
And, just for fun, they throw in this incomprehensible garbledygook:
220.127.116.11. 3600 IN A 18.104.22.168
22.214.171.124. 3600 IN A 126.96.36.199
I can't even imagine what that means.
If I ask them what IP to use directly, they say:
serifd.nl. 3600 IN A 188.8.131.52
So if I did manage to get an answer from them, it'd be wrong anyway. Your DNS is a mess.