Hey! I've had this problem for god knows how long, and I found the solution, For me at least. Go to your extension list, as it always begs you to do, and find Anything that has an update button by it. For me, it was Google Docs. It needed an update. Click the update button next to it, and you're all done. For safe measures, after you click that update button, go to the top of the page and click that update button.
It's legit and the notification and extension are built into Opera. Your only option is to click "Not now" and then it won't bug you for a while. If it still bugs you every single time after clicking "Not now", then there's something wrong.