We're having a bit of a weird problem here. It turns out we cannot access two specific websites "inktmedia.nl" and "inktmedia.com". The inktmedia.nl just forwards the request to the inktmedia.com address, but both have different ip address:
inktmedia.nl = 22.214.171.124
inktmedia.com = 126.96.36.199
The weird thing is that our DNS server can resolve these addresses just fine, but for some reason we are not able to access them. We also tried to ping these addresses from the clients, as well as the router and it's seems it's somewhat intermittent (see the screenshot).