Ah, I'm sorry,
my bad, I had obviously a f$%&up in my packet manager! Opera told me in the Update dialog it's already at latest release when it obviously wasn't. After a manual upgrade of opera-stable on the commnd line now 69.0.3686.36 is running and codecs are working fine.
Sorry for the hassle, I just wanted to understand what's the problem to be able to make a educated problem solving decision.
Thanks again for your help!