@ledsalesoz Further to this, managed to get Opera to log in by going to a thread I hadn't visited in a while and logged in on that page. Going back to the forum main page dropped me into the non-logged in screen again. So, closed the forum tab, deleted browsing history, cache etc, but problem still persists.
It seems as if, for the main page of the forum site, Opera simply refuses to reload the page from the remote server and instead is caching it from somewhere, even though FF has no such problem.