I have to go through a translator, sorry.
I’m only a small fan of Qubes OS and know almost nothing about the technical side, so I can’t give you all the information you need.
Since version 4.2.0-rc3 (I’ve just tested 4.2.0-rc4 with the same problem), installing the Mullvad VPN as described here: Using Mullvad VPN in Qubes results in an error message every time you try to connect, saying that the connection (whatever server you’re testing) is impossible.
As I said earlier, my technical background is so inadequate that I only know this configuration, but even though everything works with version 4.1.2, I can no longer use Qubes, which I love.
I can confirm that Wireguard VPN setup, in addition to being the most solid, consistent and robust implementation of Qubes+VPN I’ve used - also works flawlessly if you backup & then restore in another environment.
Everything just works. Like nothing changed. Even any firewall rules applied (gui or qvm-firewall) persist.
In past with other methods I’ve had to set things up again. But this method was a nice welcomed benefit.