I've just run into a problem and managed to fix it but it took me over an hour to figure this out. I am posting here in order to help other people that encounter a similar problem. I have recently tried to activate 2FA on my Opera account from within my Opera browser (under Windows 10). When I tried to get the QR code for my authenticator app, the code was blank *(see screenshot).

I couldn't get 2FA to work, even when I tried to type out the code manually into my authenticator app. I tried reloading the page, but could not get a QR code to render in the Opera Browser. I'm not sure what is stopping the Opera browser from rendering the QR code. My last ditch effort was to try to log into the Opera account from a different Chrome-based browser (on the same computer). From here I noticed that the QR code was visible and I was successfully able to generate a 2FA code for my account.