Why does my PySide6 DBUS ScreenSaver Signal-Listener not work?

I am currently trying to connect to the Screensaver Signal (org.freedesktop.ScreenSaver - /ScreenSaver - ActiveChanged(bool)) that is emitted when the Screen-Saver is closed:

$ dbus-monitor "interface='org.freedesktop.ScreenSaver'"
...
signal time=1761396762.341364 sender=:1.21 -> destination=(null destination) serial=94408 path=/ScreenSaver; interface=org.freedesktop.ScreenSaver; member=ActiveChanged
   boolean true
...
signal time=1761396765.026613 sender=:1.21 -> destination=(null destination) serial=94464 path=/ScreenSaver; interface=org.freedesktop.ScreenSaver; member=ActiveChanged
   boolean false

For the past few days, I have been banging my head against the wall trying to implement a DBUS Signal Listener for this signal using PySide6/Qt. This is my current attempt (combining multiple different attempts):

from __future__ import annotations

from PySide6 import QtDBus, QtCore
from PySide6.QtCore import Slot, QObject

from typing import Annotated, get_type_hints

class DBusEventDispatcher(QtCore.QObject):
    def __init__(self):
        super().__init__()
        sessionBus = QtDBus.QDBusConnection.sessionBus()
        
        if not sessionBus.isConnected():
            errorMessage = sessionBus.lastError().message()
            raise Exception("Cannot connect to DBUS: " + errorMessage)
        
        success = sessionBus.registerService("my_app")
        print(success)
        
        success = sessionBus.registerObject(
            '/', 
            'org.freedesktop.ScreenSaver',
            self,
            QtDBus.QDBusConnection.ExportAllSlots
        )
        print(success)
        
        #for path in ['org.freedesktop.ScreenSaver', 'org.gnome.ScreenSaver']:
        #    interface = QtDBus.QDBusInterface(path, '/ScreenSaver', '', sessionBus)    
        #    if interface != None:
        #        interface.ActiveChanged.connect(self._onActiveChanged)
        
        mo = self.metaObject()
        for m in range(mo.methodOffset(), mo.methodCount()):
            print(mo.method(m).methodSignature())
        
        self.iface = QtDBus.QDBusInterface(
            "org.freedesktop.ScreenSaver",  # Service
            "/ScreenSaver",                 # Path
            "org.freedesktop.ScreenSaver",  # Interface
            sessionBus
        )
        
        print(self.iface.isValid())
        
        self.iface.connect(
            QtCore.SIGNAL("ActiveChanged(bool)"), 
            self.ActiveChanged
        )
        
        QtCore.QObject.connect(
            self.iface,
            QtCore.SIGNAL("ActiveChanged(bool)"),
            self.ActiveChanged
        )
        
        # for service in ['org.freedesktop.ScreenSaver']: # 'org.gnome.ScreenSaver'
        success = sessionBus.connect(
            'org.freedesktop.ScreenSaver', # service,
            '/ScreenSaver',
            'org.freedesktop.ScreenSaver',
            'ActiveChanged',
            self,
            QtCore.SLOT('ActiveChanged(bool)')
        )
        print(success)

    @QtCore.Slot(bool)
    def ActiveChanged(self, active: bool):
        print("ActiveChanged")
        print(active)

And I get positive return values for the different calls:

$ python3 my_app.py
True
True
b'ActiveChanged(bool)'
True
True

But after that, nothing... When I lock the screen, I see the message from dbus-monitor from above, but nothing from the python script.

Does anybody know what may be wrong with the script?

Could this be a system-issue, like some systemd-rule blocking the access?

Is there maybe some debug-flag that I could set to see what is happening inside the python / DBUS library?

Any other ideas?

Thanks for any help.

$ python3 --version
Python 3.12.3
$ pip3 list
Package            Version
------------------ -------
cysystemd          2.0.1
pip                24.0
PySide6_Essentials 6.10.0
shiboken6          6.10.0
systemd            0.17.1
systemd-python     235
$ uname -a
Linux gerrit-framework 6.14.0-112033-tuxedo #33~24.04.1tux1 SMP PREEMPT_DYNAMIC Tue Sep 30 19:33:36 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
$  cat /etc/issue
TUXEDO OS 24.04.3 LTS \l

• KDE-Plasma-Version: 6.4.5
• KDE-Frameworks-Version: 6.17.0
• Qt-Version: 6.8.2
• Graphics-Platform: Wayland