A reinstalação do pacote gnome-icon-theme-symbolic
resolveu o problema. Graças à dica deste tópico: link
De apt-cache show gnome-icon-theme-symbolic
:
Description-en: GNOME desktop icon theme (symbolic icons) This package contains a set of icons used by the GNOME desktop. The icons are used in the panel menu, and in nautilus and other applications, to represent the different applications, files, directories, and devices.