Doesn’t @direc85 's last remark indicate that it’s not something to do with the connection (ip, isp, etc), but rather something with the browser?
Another site where this seems to be really easy to trigger is https://www.buienradar.nl (dutch weather service). If you’re lucky you can just about make it to the weather for a given city, but after a page refresh it goes to an ‘access denied’ page.
The same holds for me. While my phone gets an access denied page, it’s perfectly accessible from my desktop. As this is the same network, this really seems to point more to a problem with the browser.
With no other browsers available this makes a small set of sites unusable and inaccessible from sailfish. I really wish Jolla would look into this problem more closely.