Backup and restore

We ran into a similar issue on one of our testing site, and doing the following steps helped:

1. Clearing the squid cache seems to fix it (see http://drupal.org/node/202579)

2. According to this discussion thread, sometimes it might be related to DB overload (http://drupal.org/node/142684)

3. Sometimes this is caused by server issue, try to clean up your temp folder, as it might have reached its capacity.

Hope this helps...

Wen