This has occurred several times, see the attached screen capture Fix: There are many such applications that are tied to the system or admin menus that are giving backend errors. A good chance this one it too. Patch attached with submission follows: How-To-Repeat: From the system->administration menu select the menu item users and groups
For bugs matching the following criteria: Status: In Progress Changed: (is less than) 2014-06-01 Reset to default assignee and clear in-progress tags. Mail being skipped