Hoi @Maarten,
Generally, check these things:
- you're sure the internet addresses are correct
you've cleared browser cookies and cache
you've disabled any VPN you might be running, or (indeed) the KPN Malware filter
If all these things are in order, and you still can't access one or more websites, check if you can access them through mobile data. If the errors also show on mobile, the fault is most likely with the website itself.
Then it is in hands of te Operators ... as you have asked.
PS. Here it is working without problems