When trying to cancel shutdown mode via

Manage->Cancel Shutdown
or
Cancel link of Build Queue

Jenkins shows

POST is required for jenkins.model.Jenkins.doCancelQuietDown

together with a Try POSTing button.

If the Try POSTing button is pressed cancel is working with CSRF protection disabled.

But with CSRF protection enabled (in Manage->Configure Global Security) after the Try POSTing button Jenkins shows

Invalid Crumb

error which effectively means there is currently no way to cancel from shutdown mode if CSRF protection is enabled.