Well I'm not sure what changed exactly but updating Vivaldi actually worked! I can honestly say that is the first time, ever, that I've found a browser update actually fixed something, instead of breaking it (which is exactly why I turned off auto-updating!).
I guess I should feel silly? No. No I don't, as I really effing hate updates and how the break or just change things for the sake of it. Heck it was probably an update that broke Youtube in the first place... Anyway, all fixed now 🙂
Thanks for the reply and help. That's another reason I'm transitioning away from Opera; their support sucks 😉