Needs some sort of password?
I used any passwords i could think of and it couldn't work. It just gave me the same prompt after I typed one until it gave up and showed me an error notification. What do I type in the box?

Just pick another server, there seems to be an issue with that.
I picked all the servers the terminal client gave choices to and it still asked for a password.
I am having the same issue and have picked different servers. This is with using
Proton VPN CLI v3.12.0 (protonvpn-nm-lib v3.11.0; proton-client v0.7.1)
Possibly related: https://github.com/ProtonVPN/linux-app/issues/77.
This is not a Linux-specific problem, it happens in iOS too. I was able to workaround this by disconnecting the connecting server and then connecting it again. You may have to wait for a few seconds before connecting, and may have to do with 2-3 times.