@Patrick Thanks for starting to look into it.
I have found another major feature of XFCE that is also broken due due to the current whonix mod:
When you attach a VDI in VirtualBox to your Whonix-Workstation (as a way to easily expand your storage), you can no longer mount it via GUI in Thunar.
A hard disk item in Thunar appears under Devices (e.g. ‘200 GB Volume’), and when clicking on it, a GUI password prompt should appear to mount it, but nothing does in the current Whonix 15 OVA.
You have to turn off password security in the Thunar and/or UDisks2 policykit files in order to get it to work.