This can happen due to an not updated icon cache of GTK.
This can be fixed with the command
gtk-update-icon-cache, which rebuilds the GTK+ icon cache.
So why don't do this automatically in the background on a daily basis (during the night)?
Install instructions
Just execute the following commands, which will put a script into /etc/cron.daily so that the cache is fixed and the missing icon appears overnight.sudo -i echo '#!/bin/sh # # for theme in $(find /usr/share/icons -mindepth 1 -maxdepth 1 -type d) do if [ -f "$theme/index.theme" ] then gtk-update-icon-cache -f -q "$theme" fi done exit 0' > /etc/cron.daily/update-icon-cache chmod a+x /etc/cron.daily/update-icon-cache