I had the same problemin Xubuntu.. thomas-rager suggested the following on a thread for problems in twitter videos. I followed the instructions and everything is back to normal:
"H.264/MP4 problem in Opera 49.0.2725.5

Ok, 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 older version as replacement:

or as direct download:

download the file and use:
sudo dpkg --install chromium-codecs-ffmpeg-extra_61.0.3163.100-0ubuntu0.16.04.1306_amd64.deb
to install it, or double click on the file. 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

thomas-rager' suggestion.
Give it a try!