OS is LM18, opera package installed opera-stable_40.0.2308.62_amd64.deb
If you follow the rabbit hole at https://deb.opera.com, you'll see that the paths are different than expected. What exists (https://deb.opera.com/opera-stable/opera/dists/stable/non-free/binary-amd64/) is different than what the additional source the deb package creates. Guessing some linking needs to be done on your (Opera's) end.
Err:16 https://deb.opera.com/opera-stable stable/non-free amd64 Packages
404 Not Found
Ign:17 https://deb.opera.com/opera-stable stable/non-free i386 Packages
Ign:19 https://deb.opera.com/opera-stable stable/non-free all Packages
Ign:20 https://deb.opera.com/opera-stable stable/non-free Translation-en_US
Ign:21 https://deb.opera.com/opera-stable stable/non-free Translation-en
Fetched 94.5 kB in 4s (20.5 kB/s)
Reading package lists... Done
W: The repository 'https://deb.opera.com/opera-stable stable Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch https://deb.opera.com/opera-stable/dists/stable/non-free/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.