The accepted answer is actually wrong. As I explain in more details HERE, the real problem is that the path to the official Opera repositories have changed but the Opera DEB installation packages weren't reconfigured to reflect this change.
The change in repo location affects those who already were Opera users. And the lack of reconfiguration of the DEB installation packages affects new Opera users.
In order to fix this issue, all you have to do is to execute these commands at the shell terminal:
If you're using Opera stable:
sudo sed -i -e 's,opera-stable/ stable,opera-stable/opera/ stable,' "/etc/apt/sources.list.d/opera-stable.list" ; sudo apt-get update ; sudo apt-get check
If you're using Opera Beta stable:
sudo sed -i -e 's,opera-beta/ stable,opera-beta/opera/ stable,' "/etc/apt/sources.list.d/opera-beta.list" ; sudo apt-get update ; sudo apt-get check