Gnome starts but desktop icons and panels are missing

Issue

• The Gnome GUI starts, but no desktop icons or panels are showing (for all users or only non-root users).

• The following messages are also seen in the ~/.xsession-errors log:

  gnome-session[3609]: WARNING: Unable to find provider 'metacity' of required component 'windowmanager'
  gnome-session[3609]: WARNING: Unable to find provider 'gnome-panel' of required component 'panel'
  gnome-session[3609]: WARNING: Unable to find provider 'nautilus' of required component 'filemanager'
  

• After installing a new package, such as the savant-genome-browser, the graphical environment does not load completely, and shows the messages above.