I turned off two-factor authentication and now I can see the flow.

Got my Opera on Android automatically updated to 81.5.4292.78980 and tried again, this time after confirming that I'm logged in on both machines and that sync is on. Clicking the flow button in Opera for Windows (not updated since last post) prompts me to connect the phone although I'm already logged in and synced on both machines. On the phone, I was prompted again to follow the instructions on the other device (Windows) with the unchanged confirmation code shown.. This time, on Windows, I'm not getting the spinning wheel but a message "Congratulations! You're now logged in. What would you like to do?". There are two buttons; "Synchronize" and "Manage your subscriptions". I clicked the former but nothing happened. After a while got "failed to connect" on the phone. It also turned out that, without notice, sync had been switched off and I had been logged out. Looks like a problem on the Windows version since I was not logged out on the Android version.

I have the same problem with version 109.0.5097.38. Registering the phone by scanning the QR code on the Windows Opera doesn't work either. After clicking "reveal QR code" or "connect phone" and scanning the QR code with Android Opera Mobile 81.3.4292.78688, the phone displays a confirmation code (same code every time), tells me to "Follow the instructions on your other device" (Windows), but there are no instructions shown. Although I was logged in, I'm getting the spinning wheel, and it turned out that I had been logged out without notice. I logged in again (2-factor with Google Authenticator) and every time I'm getting an Email saying that it's a "new" log in, although it is from the same machine with the same IP number. Also, my synchronization turns off without notification although I had checked "sync everything" in the settings. Settings don't seem to be saved. I was logged out on my phone too, and I can't log in because "An error occurred", not very informative...