I had the same problem with the2021.1 version of mullvad.
After updating mullvad when you open up qbittorrent and go to settings-advanced-->optional IP address to bind to- ->you click the window drop down and it only shows allipv4 addresses and all ipv6 addresses.
It doesn't show you your ip address to select it, hence the red plug at the bottom of qbittorrent showing that your not connected.

Only solution I have found at the moment is to uninstall the latest version and reinstall version 2020.7. But make sure you disable the option always require VPN from the app first (if you have it enabled) before you uninstall version 2020.1
Then install version 2020.7 as normal and it should work fine as it has for me and qbittorent works fine and shows the ip address in the advanced settings, which gives you the Green plug at the bottom of the qbittorrent

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if anyone has a better solution to make 2021.1 work with qbittorrent then let us know