Version of Windows (and I don't mean just "Windows 10", but which actual build) and of Opera? I have 4 versions of Opera installed (Developer, Beta, Stable and GX) and they don't fight over which is default (Developer is). Oh, also which type of install (All users, current user, or standalone)?
@leocg I mean that when I opened up Opera GX before this issue was a thing, it updated itself automatically. I can't update or install it now, given it's uninstalled. Every single time I try to install it, it say it's trying to update the app and failing.
You could launch a command prompt and run:
"C:\Program Files\Opera\launcher.exe" --makedefaultbrowser
If that doesn't work, you can right-click the command prompt shortcut, run it as administrator and then do the same command.