XFCE Panel Preferences causes panel to restart

Today I updated my Arch Linux desktop, which included updates to some
XFCE components. Later I noticed an issue with the XFCE panel. This
not necessarily was caused by today's update as I also updated a few
days ago.

Description of the issue:
If I right-click on the panel, then choose 'Panel', then 'Panel
Preferences...', the panels (I have two) immediately restart, and I am
not shown the Preferences dialogue box.
If on the terminal I enter 'xfce4-panel p 1' to show the Preferences,
the panels also restart, but in this case there is a popup dialogue
whose screenshot I will try to send as attachment (a GDBus error,
Remote peer disconnected).

The version of the panel is 4.20.0. My installation is somewhat
non-standard as I use openbox as WM and don't have xfdesktop installed
(saying this in the unlikely event this matters).

Any ideas?

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