@leocg No, as I only use the extension on Opera, after verifying that the files were downloadable and the extension worked on other sites, I thought it might be an Opera problem and thus I came here.
As of now, it's gone back to working. Perhaps YT threw up some new change and the extension-makers have already adapted? I honestly don't have a clue. But it's the first time this has happened in literally YEARS of usage, so I was thrown.
I'll keep you up to date, I guess.