Steps to reproduce:

1. Select "Jenkins" (drop-down arrow) -> "Manage Jenkins" -> "Prepare for Shutdown" (see https://issues.jenkins-ci.org/browse/JENKINS-38175?focusedCommentId=273918&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-273918)
2. Click "cancel" link (left pane)

Result: I get "POST is required for jenkins.model.Jenkins.doCancelQuietDown" (/cancelQuietDown). Clicking "Try POSTing" gives the 403 error:

Problem accessing /cancelQuietDown. Reason: No valid crumb was included in the request