Hello Everyone, I was having the same problem, but I think I might have found a solution. From my understanding, it is something to do with the API to Windows that Opera uses to set itself as the default browser causing the issue. However, Chrome has fixed this issue with the API. Therefore, I set Chrome as the default through Windows Settings > System > Default Apps, then opened Opera and selected 'No' when asked if I wanted to set Opera as the default, and checked not to show this message again. I then closed Opera, opened Chrome, navigated to any web page, then closed Chrome. I then went back into Windows Settings and set Opera as the default, opened Opera and NO SETTING POPPED UP!!
Hopefully this will work for you too! - Please let me know if it does and I will post the solution on other forums to help others having the same issue.
Worked great for me too! (I did reset to Microsoft defaults in the Default Apps page before doing all the steps mentioned) Thanks so much!