This is the message I got every time I want to log into Google Docs or Slides. Cookies are enabled, cache are cleared, I could not find anything in Opera settings related to the last point.
''We've detected a problem with your cookie settings.
Make sure your cookies are enabled. To enable cookies, follow these browser-specific instructions.
Clear cache and cookies
If you have cookies enabled but are still having trouble, clear your browser's cache and cookies.
Adjust your privacy settings
If clearing your cache and cookies doesn't resolve the problem, try adjusting your browser's privacy settings. If your settings are on high, manually add www.google.com to your list of allowed sites. Learn more''