If the Jabber plugin is installed and enabled and the Jabber server goes down,
this causes Hudson to throw an exception when submitting any change to any
plugin in the Manage Hudson -> Configure System page. If the Jabber server
cannot be started right away, the only remedy is to go into the plugin
configuration, un-check the Jabber plugin, and restart the Hudson server.

Other plugins such as the IRC plugin provide a checkbox on the Configure System
page which allows the administrator to enable or disable the plugin dynamically
without restarting Hudson. Such a checkbox would be very useful in the Jabber
plugin.