It's because your vpn exit node was blocked by RT. You share IPs with other mullvad customers. Could be becaue someome was hacking or just hitting a rate limit. Try again later or use another exit node.

@Dr_Vindaloo Mullvad did not. The website blocked Mullvad.

I do not use Mullvad, however, I have had the same issue with sites, not rt(.com) in particular, but others. All I do is switch locales and usually the problem clears up. I haven't figured out exactly why. A lot of VPN IPs have a tendency to be abused and so a certain website may have blacklisted your IP number of the VPN locale you were/are using. Could be some scenario such as this.

Also, if you use Mullvad's DNS servers, some level-1 DNS providers block Mullvad's level-2 servers, which means you can't even look up some sites. My solution was to use a web DNS lookup tool and add blocked resolves to my router's dnsmasq table. It's a shitty solution, but it works.