For anyone that is interested, there is a solution to this problem up on the AUR site:
AUR (en) - nordvpn-bin (second comment)
Basically it involves downgrading nordvpn-bin to an earlier version, the link explains how.
In my case I also had a missing dependency called ipset which I had to install before running makepkg.
It all seems to be working again now although I haven’t been running it for long.