Any explaination about this?

QUIC is quite interesting https://en.wikipedia.org/wiki/QUIC

So.. Dutch ISPs - or someone else along the route - interfering with UDP traffic?

I suspect they are throttling UDP traffic on "non-web" ports, traffic might look like torrent encrypted stream :). Anyway you can test whats going on with https://github.com/lucas-clemente/quic-go