I have the same problem as described herein:
"After using Firefox for many moons, I've switched to Opera (again), as it runs one heck of a lot better these days. I have set Opera as my default browser, and to always check that it's the default. But now, my Windows 10 default settings page pops up automatically whenever Opera loads! Even though Windows has Opera listed as the default browser. Opera's settings do not give me any way to stop that unnecessary settings page from loading."
So it's not resolvable, well it's not the end of the world (I don't think).