Reading this discussion I've thought that it worth a separate thread.
As it said in the link above Opera for some unknown reasons does not allow injecting content scripts on extensions gallery pages (https://addons.opera.com/). Google Chrome and Firefox allow it on their extensions gallery pages. It is a bit strange, I think, because first, I didn't find that fact in documentation, and second if it is forbidden why the https://addons.opera.com/ does not programmatically excluded in <all_urls> matches?
The issue is that if this domain is missed by developer in a testing process that means only after submitting his/her perfect (as they think) extension they will discover the red color error right after the first install of an extension from... the page where scripting is forbidden. BINGO! All the rest of the world is allowed to be scripted by Opera but this particular place not.
Another part of this issue is that a user is informed on the extension page that something will happen after installing it but nothing happens and some impatient users may even uninstall actually working extension at once. BINGO! )):
What we need I think to rid of this restriction in Opera or clearly reveal that security paranoid in the Opera documentation and all the https://addons.opera.com/* pages for users.