This issue is caused when you disable one of the CloudBees DEV@cloud plugins. Try to re-enable or re-install the CloudBees Credentials Plugin.
Update (2014-01-08)
A newer root cause is applicable if your Jenkins instance was hibernated or restarted any time after mid-November 2013. For these cases:
The update of DEV@cloud to the cloudbees-folder
plugin version 4.0 (which only kicks in when your Jenkins instance is restarted after Nov 13th 2013) requires that the credentials
plugin be upgraded to the latest (at least 1.9.3) and the ssh-credentials
and ssh-slaves
plugins be upgraded to the latest versions and Jenkins restarted.