@jimunderscorep I'll try my best here but it might be not understandable because English is not my first language.
When I installed Opera, I did install it as a regular app (.deb from official website). But with your last reply I realized that the version of Flash that was installed was the 32 bit, so I removed it and installed the 64 bit one. Then I removed chromium ffmpeg codecs from Snap since I installed Opera as a regular app.
When I tried to install chromium ffmpeg codecs with apt, I got this: "chromium-codecs-ffmpeg-extra is already the newest version (74.0.3729.169-0ubuntu0.19.04.1)" so it was already installed before I install Opera.
Opera is installed as a regular app;
Chromium ffmpeg codecs was already installed;
Flash now is installed correctly.
But I got the same error.
I tried this, though:
1 - Remove Opera as a regular app;
2 - Install Opera and chromium ffmpeg codecs with Snap;
The result was: (predictable since it was the Snap version and you told me that a Snap app is run sandboxed)
Twitch.tv worked fine, but here (https://helpx.adobe.com/br/flash-player.html) Flash was undetectable and websites that uses Flash didn't work (there was a green button telling "Install Flash").