Possibly missing wayland support
Wayland is a server display protocoll with more security features then Xorg and the aim to become the successor of Xorg. Maybe I am just not experienced enough, but it seems, that riseup-vpn from the AUR can only be started under Xorg or xwayland. So there is a need to have Xorg or xwayland and renounce the security features of wayland. I am not sure, why I can not start it in Wayland, because Riseup-VPN uses QT which supports Wayland with the environment variable "QT_QPA_PLATFORM=wayland".
The only option to use Riseup-vpn is so the deprecated riseup-vpn red at the moment: https://riseup.net/fr/vpn/vpn-red/linux Maybe a command line version could be a solution for a working vpn on much more systems.
More about wayland: https://wiki.archlinux.org/index.php/Wayland
If it is possible to start riseup vpn without x-server please excuse my false Reporting and maybe you could explain me how.