The simplest solution is to completely disable security - change true to false in /var/lib/jenkins/config.xml file.
Then just restart Jenkins, by
sudo service jenkins restart
And then go to the admin panel and set everything once again.
If you are running your Jenkins inside k8s pod from a docker, and can not run service command, then you can just restart Jenkins by deleting the pod:
kubectl delete pod <jenkins-pod-name>
Once the command was issued, the k8s will terminate the old pod and start a new one.