Main window does not open (Debian Trixie, KDE, v33.0.0)

The client in version 33.0.0 seems to work fine, except that the "main window" can't be opened. This is the window I mean:

Using "Open Nextcloud Desktop" from the system tray icon's context menu (right mouse button click) does not work either.

This worked fine in 4.0.8 and it also works when using the AppImage version 33.0.0, which is why I create the issue here. It seems to be flatpak-specific in some way.

This is what I see on the terminal when starting the flatpak app from there:
```
$ flatpak run com.nextcloud.desktopclient.nextcloud
Qt: Session management error: Could not open network socket
nextcloud.gui.application: Migrating old config from "/home/schlimmchen/.var/app/com.nextcloud.desktopclient.nextcloud/data/Nextcloud" to "/home/schlimmchen/.var/app/com.nextcloud.desktopclient.nextcloud/config/Nextcloud"
nextcloud.gui.application: Failed to move the old config directory to its new location ( "/home/schlimmchen/.var/app/com.nextcloud.desktopclient.nextcloud/data/Nextcloud" to "/home/schlimmchen/.var/app/com.nextcloud.desktopclient.nextcloud/config/Nextcloud" )
nextcloud.gui.application: Will move the individual files QList("nextcloud_sync.log")
nextcloud.gui.application: Fallback move of "nextcloud_sync.log" also failed
```

System Info:
```
Operating System: Debian GNU/Linux 13
KDE Plasma Version: 6.3.6
KDE Frameworks Version: 6.13.0
Qt Version: 6.8.2
Kernel Version: 6.12.73+deb13-amd64 (64-bit)
Graphics Platform: X11
```