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For a more precise explanation: the moment I allow qBit on the firewall, anything closely related to online gameplay stops working. No games on Steam would allow me to even log-in into multiplayer sessions, nor other non-Steam games. I'm guessing this is some sort of a port-forwarding issue, but I don't understand why qBit would prevent other programs from accessing online features. The multiplayer functions start working again only once I set my firewall to default settings - hence why I know this is a firewall-related issue. Any clues folks?

WTF that is SO weird.
I think you might be using a broken firewall? What program do you use?

It's just Window default -- could be Bitdefender that is causing the issue, not the firewall itself. But in any case, resetting the firewall settings make things work again. Weirdest issue I've ever had since I've torrented lols

Bitdefender also has a firewall and an AV.
Honestly, just use Windows default + common sense.