I can confirm:
sudo mv /usr/share/gdm/autostart/LoginWindow/metacity.desktop \ /usr/share/gdm/autostart/LoginWindow/metacity.desktop.backup
supresses the symptoms, an error message no longer pops up.
I can confirm:
sudo mv /usr/share/ gdm/autostart/ LoginWindow/ metacity. desktop \ gdm/autostart/ LoginWindow/ metacity. desktop. backup
/usr/share/
supresses the symptoms, an error message no longer pops up.