This seems very similar to a recently closed #57143. We have spent quite a bit of time troubleshooting it though not able to find the root case. We had nginx and replace that by elb and both cases leads to same problem below. Please let me know what additional logs/info are needed and will provide. Any help/support/guidance is appreciated.

1. netcat from slave towards master. 

$ nc -z jenkins.infrastructure.XXXXX.care 50000

Connection to jenkins.infrastructure.XXXXX.care port 50000 [tcp/*] succeeded!

#ELB

kubectl -n infrastructure get svc
infrastructure-nginx-ingress-internal-controller LoadBalancer 100.66.210.45 internal-ab39ee254472111e9aa5b0a4f0f93d47-951468028.us-east-1.elb.amazonaws.com 80:30664/TCP,443:30937/TCP,50000:32123/TCP 166d

1. Logs

Aug 31, 2019 7:39:48 PM hudson.remoting.jnlp.Main createEngine

INFO: Setting up agent: ICL-Slave

Aug 31, 2019 7:39:48 PM hudson.remoting.jnlp.Main$CuiListener <init>

INFO: Jenkins agent is running in headless mode.

Aug 31, 2019 7:39:48 PM hudson.remoting.Engine startEngine

INFO: Using Remoting version: 3.29

Aug 31, 2019 7:39:48 PM org.jenkinsci.remoting.engine.WorkDirManager initializeWorkDir

INFO: Using /Users/vmccbuild/jenkins/remoting as a remoting work directory

Aug 31, 2019 7:39:48 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Locating server among https://jenkins.infrastructure.xxxxx.care/

Aug 31, 2019 7:39:48 PM org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver resolve

INFO: Remoting server accepts the following protocols: [JNLP4-connect, Ping]

Aug 31, 2019 7:39:48 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Agent discovery successful

  Agent address: jenkins.infrastructure.xxxxx.care

  Agent port:    50000

  Identity:      6b:c4:c2:2b:ed:b7:da:bc:72:58:c8:83:dd:d0:67:46

Aug 31, 2019 7:39:48 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Handshaking

Aug 31, 2019 7:39:48 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Connecting to jenkins.infrastructure.xxxxx.care:50000

Aug 31, 2019 7:39:49 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Trying protocol: JNLP4-connect

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Protocol JNLP4-connect encountered an unexpected exception

java.util.concurrent.ExecutionException: org.jenkinsci.remoting.protocol.impl.ConnectionRefusalException: Connection closed before acknowledgement sent

at org.jenkinsci.remoting.util.SettableFuture.get(SettableFuture.java:223)

at hudson.remoting.Engine.innerRun(Engine.java:614)

at hudson.remoting.Engine.run(Engine.java:474)

Caused by: org.jenkinsci.remoting.protocol.impl.ConnectionRefusalException: Connection closed before acknowledgement sent

at org.jenkinsci.remoting.protocol.impl.AckFilterLayer.onRecvClosed(AckFilterLayer.java:280)

at org.jenkinsci.remoting.protocol.FilterLayer.abort(FilterLayer.java:164)

at org.jenkinsci.remoting.protocol.impl.AckFilterLayer.access$000(AckFilterLayer.java:43)

at org.jenkinsci.remoting.protocol.impl.AckFilterLayer$1.run(AckFilterLayer.java:176)

at org.jenkinsci.remoting.protocol.IOHub$DelayedRunnable.run(IOHub.java:964)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)

at hudson.remoting.Engine$1.lambda$newThread$0(Engine.java:93)

at java.lang.Thread.run(Thread.java:748)

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Connecting to jenkins.infrastructure.xxxxx.care:50000

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Server reports protocol JNLP4-plaintext not supported, skipping

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Server reports protocol JNLP3-connect not supported, skipping

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Server reports protocol JNLP2-connect not supported, skipping

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener status

INFO: Server reports protocol JNLP-connect not supported, skipping

Aug 31, 2019 7:39:59 PM hudson.remoting.jnlp.Main$CuiListener error

SEVERE: The server rejected the connection: None of the protocols were accepted

java.lang.Exception: The server rejected the connection: None of the protocols were accepted

at hudson.remoting.Engine.onConnectionRejected(Engine.java:682)

at hudson.remoting.Engine.innerRun(Engine.java:639)

at hudson.remoting.Engine.run(Engine.java:474)