Small (and final) update.
Turns out it was something with the vivaldi itself. No idea what, but out of curiousity i simply reinstalled the browser and tried again.
Worked like a charm. The only difference with the setup i made was with the trackers and ads, which i put into "no protection" this time around. Just to be "safe".
So, either it was that option, which for some reason didn't update when i changed it in settings first time around, or it was something else, which simply broke and didn't bother informing me about it.
Anyway, problem solved. Thanks again for your time 🙂