This is a easy fix!
Download the last libffmpeg from here and decompress it.
Install at Opera directory, click at the top left corner at the Opera logo and About Opera, look at your Opera Installation path, on Debian is at
Copy the libffmpeg.so to that path with:
sudo cp libffmpeg.so /usr/lib/x86_64-linux-gnu/opera
The fix is not permanent, for every Opera update you need to do this again.
For Debian there is no chromium codec and this libffmpeg is always up-to-date and align with Opera updates.
Works! Opera 54.0.2952.41 (Ubuntu 18.04)