Created attachment 3955 fix gtk_message_dialog_new_with_markup() usage gtk_message_dialog_new_with_markup() in panel-plugin/xfce4-clipman-settings.c is used incorrectly, it is a variadic function, the trailing NULL pointer thus needs to be removed. See attached patch for a fix.
Thank you, fixed in git 0f0a0eb..b7586d0.