Linux agents are not starting anymore

$ cat Jenkins\ Prebuilt\ Slave\ \(sir-p9ai6v8m\)/slave.log
Oct 08, 2018 10:43:49 PM hudson.plugins.ec2.EC2Cloud
INFO: Launching instance: null
Oct 08, 2018 10:43:49 PM hudson.plugins.ec2.EC2Cloud
INFO: bootstrap()
Oct 08, 2018 10:43:49 PM hudson.plugins.ec2.EC2Cloud
INFO: Getting keypair...
Oct 08, 2018 10:43:49 PM hudson.plugins.ec2.EC2Cloud
INFO: Using key: master
xxx
-----BEGIN RSA PRIVATE KEY-----
xxx
Oct 08, 2018 10:43:49 PM hudson.plugins.ec2.EC2Cloud
INFO: Authenticating as ubuntu
ERROR: Unexpected error in launching an agent. This is probably a bug in Jenkins
java.lang.NullPointerException
	at hudson.plugins.ec2.ssh.EC2UnixLauncher.getEC2HostAddress(EC2UnixLauncher.java:368)
	at hudson.plugins.ec2.ssh.EC2UnixLauncher.connectToSsh(EC2UnixLauncher.java:318)
	at hudson.plugins.ec2.ssh.EC2UnixLauncher.bootstrap(EC2UnixLauncher.java:282)
	at hudson.plugins.ec2.ssh.EC2UnixLauncher.launchScript(EC2UnixLauncher.java:130)
	at hudson.plugins.ec2.EC2ComputerLauncher.launch(EC2ComputerLauncher.java:48)
	at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:294)
	at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
	at jenkins.security.ImpersonatingExecutorService$2.call(ImpersonatingExecutorService.java:71)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)