Debian 9: USB-volumes are not displayed in PCManFM-Qt's sidebar

I run Debian 9 with LXQt what is completely fine. However, attached USB-volumes are not getting displayed in the PCManFM-Qt sidebar in category Devices intended for that (see screenshot). Which package is responsible for that? I activated all three necessary options in Preferences -> Volume, but the drives still don't appear in the sidebar. Ironically the taskbar-widget shows the drives and I can easily open them in PCManFM-Qt by clicking on the widget. However they are never displayed in the sidebar while the category Devices remains completely empty. During common workflows that is a bit uncomfortable... ---------- **Screenshot of the issue:** **Screenshot of the working taskbar widget:** PCManFM-Qt correctly" class="img-fluid rounded" style="max-width: 100%; height: auto; margin: 10px 0;" loading="lazy"> Again, clicking on the taskbar widget the device gets opened in PCManFM-Qt without any problems. Only displaying its existence in the sidebar does not work.