I use Jenkins ver. 1.594 with groovy-postbuild-plugin v2.2 but it's failed on project configuration with message "startup failed:
Script1.groovy: 1: unable to resolve class PostbuildActions". PostbuildActions.groovy is in /var/lib/jenkins/workspace/Checkout_jenkins/groovy-postbuild-actions. It's readable and executable. I have tried to select "Use Groovy Sandbox" option but it didn't helped me.