In case anyone else comes looking for the solution to this, I believe I've found it. The part the original poster was missing might have been deleting the HSTS policy along with the history and cache.
Open the URL opera://net-internals/#hsts
Put the affected domain name in the bottom box, "Delete domain security policies," and click the Delete button.
Open opera://history
Put the domain in the "Search history" box. If any matches come up, hover over one and "Remove entire domain"
Open opera://settings/clearBrowserData
Select More > All Time, deselect all checkboxes except "Cached images and files," and click Delete data.
Browse to your URL explicitly using http:// instead of https://
I did not need to restart Opera to make this take effect. Hope this helps someone out there who's been wasting as much time as I just did trying to figure this out.