I just had the same problem after a week of using Opera easily. Suddenly I couldn't access my gmail account, in the midst of using it. None of Google's "helps" in its error message about detecting a problem with the cookie settings helped. I did as a writer above suggested, cleared browsing history and cookies for 4 weeks, and that did the trick.
Unless it's an intermittent prank played by Google on those of us who have forsaken Chrome.