I have the exact same problem on fully updated Arch, but there's more - Widevine isn't working in the newest version of Opera, hence I downgraded. So far version 65.0.3467.69 works perfectly on newest ffmpeg-codecs.
Fyi, on every update, of any package, the package manager replaces the package's existing files with the new, upadted ones. Why should opera be an exception to it?
@leocg and @burni thanks a lot! I had searched several solutions on my transition from Mint to Fedora, and problaby I will work with Chrome when I had some streaming issues (I watch much more than Deutsche Welle Live, it is actually just a random exemple).
Maybe I try Vivaldi sometime, or get back to Firefox (I had some issues with this browser too).
Thanks folks, and I hope Opera team could resolve this problem for the linux users, because I like this browser a lot.
No idea about the warning on mint though. It may mean the playback on mint's default media player, which is probably some gstreamer based app, so it may just install gstreamer1.0-plugins-bad and gstreamer1.0-plugins-ugly. I do not use the gstreamer backend on any app, so I have no idea what each package contains and why they are called like that.
Anyhow, if it is gstreamer related, it won't affect opera and chromium.
Problem fixed. I simply copied this --start-maximized and pasted it in my task-bar shortcut leaving a space and BOOM! Fixed!Screenshot at 2019-12-31 05-21-42.png