For me, the fix was short lived. Today I am getting this again.
The repository 'https://deb.opera.com/opera-stable/opera stable Release' does not have a Release file.Data from such a repository can't be authenticated and is therefore potentially dangerous to use.See apt-secure(8) manpage for repository creation and user configuration details.Failed to fetch https://deb.opera.com/opera-stable/opera/dists/stable/non-free/binary-amd64/Packages 404 Not FoundSome index files failed to download. They have been ignored, or old ones used instead.