Cannot find any polkit agent

I've installed riseup-vpn on Arch Linux (kernel 5.11.16) but I'm unable to start it. Whenever I run riseup-vpn or sudo riseup-vpn, it shows this log:

QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
qrc:/qml/main.qml:150:13: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
QSystemTrayIcon::setVisible: No Icon set
qrc:/qml/main.qml:19:5: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
qml: systray init completed
qml: show systray
qml: Platform: linux
qml: DEBUG: Pre-seeded providers:
qml: {
    "default": "riseup",
    "providers": [
        {
            "apiURL": "https://api.black.riseup.net/",
            "applicationName": "RiseupVPN",
            "askForDonations": "true",
            "auth": "anon",
            "authEmptyPass": "false",
            "binaryName": "riseup-vpn",
            "caCertString": "-----BEGIN CERTIFICATE-----
[confidential(?)]
-----END CERTIFICATE-----",
            "donateURL": "https://riseup.net/vpn/donate",
            "geolocationAPI": "https://api.black.riseup.net:9001/json",
            "helpURL": "https://riseup.net/support",
            "name": "Riseup",
            "providerURL": "riseup.net",
            "timeStamp": "2021-02-08 23:57:09",
            "tosURL": "https://riseup.net/tos"
        }
    ]
}

2021/05/02 22:00:45 Client expects anon auth
2021/05/02 22:00:45 Cannot find any usable polkit
2021/05/02 22:00:45 no polkit
qml: ERRORS: nopolkit
qml: ERRORS: nopolkit
qml: ERRORS: nopolkit
2021/05/02 22:00:47 Close: cleanup and vpn shutdown...
2021/05/02 22:00:47 firewall stop
2021/05/02 22:00:47 openvpn stop
2021/05/02 22:00:47 Close: cleanup and vpn shutdown...
2021/05/02 22:00:47 firewall stop
2021/05/02 22:00:47 openvpn stop

(When the 3 ERRORS: nopolkit lines are logged, it also opens a dialog box saying "Could not find polkit agent"). But I do have multiple polkit agents installed (polkit 0.118-1, polkit-gnome 0.105-8 and polkit-qt5 0.113.0-2). I have no idea how to fix this, and I was unable to find any similar issue reported here or on the internet. (Side note: I also tried installing it via snapstore, but it had trouble accessing Qt plugins for some reason. However, after every reboot, it showed the same "Could not find polkit agent" dialog box, but the tray icon remained there and I was able to switch on the VPN from there.) Thanks for any help :)