Today I tried the procedure to replace the libffmpeg.so file from Chromium and it works fine. Over time only the path to the files changed. Now it is:
I deleted the libffmpeg.so file from Opera with:
sudo rm /usr/lib/x86_64-linux-gnu/opera-stable/libffmpeg.so
and copied the better one from the new Chromium:
sudo ln -s /usr/lib/chromium/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera-stable/libffmpeg.so
Thank you for this advice. I was already desperate and tried using other browsers like Firefox or Chromium, but for me Opera is the best for many practical reasons.