From JENKINS-24047:

Would also suggest somehow letting cloudbees-enterprise-plugins let you set up your license in advance of the restart, so that there is no window during which you are blocked from using Jenkins. Not sure how best to do this while offering the full functionality of both EvaluationRegistrar and EnterpriseRegistrar. Perhaps just display the instance ID, offer a link to licenses.cloudbees.com, and suggest you register via the web and manually copy back your details to Jenkins, using the equivalent of ManualRegistrar?