See this log:

From: https://serverfault.com/questions/880623/jenkins-ec2-plugin-http-error-404/882851#882851

{{Oct 27, 2017 1:26:31 PM null
FINEST: Node jenkins-slave (i-0f8aa5fb0ab29164f)(i-0f8aa5fb0ab29164f) is still pending/launching, waiting 5s
Oct 27, 2017 1:26:36 PM null
FINEST: Node jenkins-slave (i-0f8aa5fb0ab29164f)(i-0f8aa5fb0ab29164f) is still pending/launching, waiting 5s
Oct 27, 2017 1:26:41 PM null
FINEST: Node jenkins-slave (i-0f8aa5fb0ab29164f)(i-0f8aa5fb0ab29164f) is still pending/launching, waiting 5s
Oct 27, 2017 1:26:46 PM null
FINER: Node jenkins-slave (i-0f8aa5fb0ab29164f)(i-0f8aa5fb0ab29164f) is ready
Oct 27, 2017 1:26:46 PM null
INFO: Launching instance: i-0f8aa5fb0ab29164f
Oct 27, 2017 1:26:46 PM null
INFO: bootstrap()
Oct 27, 2017 1:26:46 PM null
INFO: Getting keypair...
Oct 27, 2017 1:26:46 PM null
INFO: Using key: jenkins-slave
d5:dd:51:ac:8b:b3:bf:1b:6f:b4:b1:86:5b:26:14:1e:e0:47:25:72
----BEGIN RSA PRIVATE KEY----
MIIEowIBAAKCAQEAoxetaNHULbsbqsThxUSxxxxxxxxxxxxxxxxxxxxxxxxxx
ga2jnJaFOG4M/Fnes9ZzHclz7eflwliFZnhIUJldlSBZ2bvAqdp
Oct 27, 2017 1:26:46 PM null
INFO: Authenticating as ec2-user
Oct 27, 2017 1:26:46 PM null
INFO: Connecting to 10.0.2.51 on port 22, with timeout 10000.
Oct 27, 2017 1:26:47 PM null
INFO: Failed to connect via ssh: There was a problem while connecting to 10.0.2.51:22
Oct 27, 2017 1:26:47 PM null
INFO: Waiting for SSH to come up. Sleeping 5.
Oct 27, 2017 1:26:52 PM null
INFO: Connecting to 10.0.2.51 on port 22, with timeout 10000.
Oct 27, 2017 1:26:52 PM null
INFO: Failed to connect via ssh: There was a problem while connecting to 10.0.2.51:22
Oct 27, 2017 1:26:52 PM null
INFO: Waiting for SSH to come up. Sleeping 5.
Oct 27, 2017 1:26:57 PM null
INFO: Connecting to 10.0.2.51 on port 22, with timeout 10000.
Oct 27, 2017 1:26:57 PM null
INFO: Failed to connect via ssh: There was a problem while connecting to 10.0.2.51:22
Oct 27, 2017 1:26:57 PM null
INFO: Waiting for SSH to come up. Sleeping 5.
Oct 27, 2017 1:27:02 PM null
INFO: Connecting to 10.0.2.51 on port 22, with timeout 10000.
Oct 27, 2017 1:27:03 PM null
INFO: Connected via SSH.
Oct 27, 2017 1:27:03 PM null
INFO: connect fresh as root
Oct 27, 2017 1:27:03 PM null
INFO: Connecting to 10.0.2.51 on port 22, with timeout 10000.
Oct 27, 2017 1:27:03 PM null
INFO: Connected via SSH.
Oct 27, 2017 1:27:03 PM null
INFO: Creating tmp directory (/tmp) if it does not exist
Oct 27, 2017 1:27:03 PM null
INFO: Verifying that java exists
java full version "1.7.0_151-mockbuild_2017_08_09_21_42-b00"
Oct 27, 2017 1:27:03 PM null
INFO: Copying slave.jar
Oct 27, 2017 1:27:03 PM null
INFO: Launching slave agent (via Trilead SSH2 Connection): java -jar /tmp/slave.jar

HTTP ERROR 404

Problem accessing /computer/jenkins-slave%20(i-0f8aa5fb0ab29164f)/logText/progressiveHtml. Reason:

Not Found
Powered by Jetty:// 9.4.z-SNAPSHOT}}