Thanks for all the detailed information, that helped us dig quite deep into the issues you're experiencing!
Unfortunately after looking at our front and back end logs, we believe this issue is only happening to you. Others on your ISP, don't have any issues accessing images on i.sstatic.net. We checked the CDN server you're accessing the images from, our back end server, along with trying to reproduce your errors using proxies and other devices. Ultimately we were unable to reproduce the error you're getting with any of the combinations we tested.