Command line like this:

    java -jar jenkins-plugin-manager-1.0.1.jar -f plugins.txt --verbose --plugin-download-directory ~/plugins --no-download --list --war /usr/lib/jenkins/jenkins.war --jenkins-update-center https://jenkins-updates.cloudbees.com/

fails with stack trace containing

    java.io.FileNotFoundException: https://jenkins-updates.cloudbees.com//update-center.json

Remove the trailing slash from the desired update center and it accepts the supplied update center. (This is the workaround)