See this forum post for info on recreating: http://forum.civicrm.org/index.php/topic,3964.0/topicseen.html
I think this may only affect Custom Data Groups where display is TAB (as opposed to INLINE) ?? And may not happen if ALL file upload fields have been populated.
Fatal error when tested on 2.0 demo:
warning: rename(,/var/www/drupal.demo.civicrm.org/public/sites/drupal.demo.civicrm.org/files/civicrm/custom//) [function.rename]: No such file or directory in /var/www/drupal.civicrm.org/public/sites/drupal.demo.civicrm.org/modules/civicrm/CRM/Core/BAO/File.php on line 115.