Same for me since this afternoon.
HTTP response code: 504 or 503 -> servers seems to be overloaded ?
Just wait :( or try to grab the plugins files manually and put them into
JENKINS_HOME/plugins, restart jenkins.
Kr
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I have the same problem reported by rec (deleted message)
Apr 2, 2014 2:04:11 PM hudson.model.UpdateCenter$DownloadJob run
SEVERE: Failed to install Credentials Plugin
java.io.IOException: Failed to download from
http://updates.jenkins-ci.org/download/plugins/credentials/1.10/credentials.hpi
