@leocg I'm guessing it's not all users, but as you can see for comments, it is not working for most users. Developers really need to check this issue.
I'll try to add more details in case that helps developers:
Inmediately after pressing Add to Opera, Opera appears in red in the task bar, same as when a download fails. What's happening in the background is that the download always fails and tries to restart all the time. If the user removes the extension and goes back, they can see how all the downloads of extensions are there.
This means that the extension is never added to the extensions and users are unable to ever press Install. That's also why if it's downloaded as a zip it can be manually installed.