Solved; that was quick. :-)
For other reasons, /tmp had incorrect permissions. Changing them with "chmod 1777 /tmp" solved the problem.
The hint was this post, https://www.centos.org/modules/newbb/viewtopic.php?topic_id=41813&forum=... , which described other symptoms I also had: "gconf-sanity-check-2 exited with status 256", and "The configuration defaults for GNOME Power Manager have not been installed correctly".
Thanks to jeepster for his post.