I had some time to play around. On Debian (amd64) based systems (Debian, Ubuntu, Mint, ect) you need to uninstall
chromium-codecs-ffmpeg-extra with the command:
sudo apt-get remove chromium-codecs-ffmpeg-extra
You can uses this version as replacement:
download the file and double click on the file to install it. Afterwards completely restart Opera.
If you want to prevent further updates of this package use this command (not recommended):
sudo apt-mark hold chromium-codecs-ffmpeg-extra