The Opera browser recently introduced a VPN feature.
When turned on, it tunnels traffic through their (Opera Software's) proxy.
When opening Stack Exchange pages like
Page Not Found We couldn't find the page you requested. But we haven't looked inside your computer for it. Yet.
Those pages work in direct connection as usual.
I can see the Stack Exchange page, but not the content.
I've checked the network response, and it seems that the proxy responds with 404 on
http version of URL is reached.
When I try the same URL with
https it works. So I assume it's a problem with Opera.