@burnout426 Yes, I first installed Creators Update from a DVD that I had saved somewhere else, and while installing other programs it was updated to Fall Creators Update, so I assume that was the problem. And when I couldn't find a solution, I downloaded Fall Creators Update to a USB stick and made a clean installation. After I finished installing Windows I made sure to disable IPv6 in the network properties and disabled the option to automatically detect in the proxy settings. Finally I installed Opera and started to try out the pages that were giving me problems and they worked correctly.
I know it was a desperate move, but as I could not find a solution, I had no choice but to do so. I don't know if it's the definitive solution but it worked for me. Still, I appreciate your help.