@avl Thanks! It worked.

This is pretty inconvenient though, if one has to reinstall the same piece of software multiple times for each browser. Maybe Opera should bundle Flash, like Chrome does.

I just updated to Opera 52 from Opera 51, on Mac OS X 10.13.3. Since then, every website using Flash tells me that Flash is missing or not enabled. This includes Adobe's own help website: https://helpx.adobe.com/flash-player.html. I have installed the latest version of Flash and tried running Opera with both "Allow sites to run Flash" and "Ask first before allowing sites to run Flash (recommended)" options. I've added the websites in questions to the exceptions list, both with and without http(s). I've deactivated the ad blocker as well. Nothing has worked so far. Flash works fine on Safari, Chrome and Firefox.