I was experiencing the same problem. In my case it turned out to be my web filter was blocking certain content from opera.com and opera-mini.net. Adding a rule to my web filter to allow content from both domains allowed proper display of the add-ons page.
Thanks for replying. I'm not sure I know what a web filter is. Is it something in opera settings or a separate app? Cuz if it is a separate app then I don't think I have something like that installed on my machine.