@PixCatholica, I did some research and replaced libffmpeg.so with the one I downloaded from https://packages.ubuntu.com/xenial-updates/amd64/chromium-codecs-ffmpeg-extra/download
Download the package, find libffmpeg.so in it and extract it.
Open /usr/lib/x86_64-linux-gnu (this path may vary) click with right mouse button on "opera" folder and "Open as Root". Replace the libffmpeg.so with the new file.
Now you can play videos.