I installed the jenkins package from  http://pkg.jenkins.io/debian-stable on a fresh installed Debian Buster machine using apt. After the initial configuration with the recommended plugins I added a new Node and tried to connect this agent.

Initially all looks fine, and the output of the agent.jar process tells

INFO: Connected

But a view minutes later the agent.jar process ends with the exception as shown below:

INFO: Protocol JNLP4-connect encountered an unexpected exception
java.util.concurrent.ExecutionException: org.jenkinsci.remoting.protocol.impl.ConnectionRefusalException: Remote closed connection without specifying reason
{{ at org.jenkinsci.remoting.util.SettableFuture.get(SettableFuture.java:223)}}
{{ at hudson.remoting.Engine.innerRun(Engine.java:744)}}
{{ at hudson.remoting.Engine.run(Engine.java:519)}}
Caused by: org.jenkinsci.remoting.protocol.impl.ConnectionRefusalException: Remote closed connection without specifying reason
{{ at org.jenkinsci.remoting.protocol.impl.ConnectionHeadersFilterLayer.onRecvClosed(ConnectionHeadersFilterLayer.java:440)}}
{{ at org.jenkinsci.remoting.protocol.ProtocolStack$Ptr.onRecvClosed(ProtocolStack.java:816)}}
{{ at org.jenkinsci.remoting.protocol.FilterLayer.onRecvClosed(FilterLayer.java:288)}}
{{ at org.jenkinsci.remoting.protocol.impl.SSLEngineFilterLayer.onRecvClosed(SSLEngineFilterLayer.java:172)}}
{{ at org.jenkinsci.remoting.protocol.ProtocolStack$Ptr.onRecvClosed(ProtocolStack.java:816)}}
{{ at org.jenkinsci.remoting.protocol.NetworkLayer.onRecvClosed(NetworkLayer.java:154)}}
{{ at org.jenkinsci.remoting.protocol.impl.BIONetworkLayer.access$1500(BIONetworkLayer.java:48)}}
{{ at org.jenkinsci.remoting.protocol.impl.BIONetworkLayer$Reader.run(BIONetworkLayer.java:247)}}
{{ at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)}}
{{ at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)}}
{{ at hudson.remoting.Engine$1.lambda$newThread$0(Engine.java:118)}}
{{ at java.base/java.lang.Thread.run(Unknown Source)}}
{{ Suppressed: java.nio.channels.ClosedChannelException}}
{{ ... 7 more}}

Mõrz 02, 2021 2:00:31 NACHM. 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:829)}}
{{ at hudson.remoting.Engine.innerRun(Engine.java:769)}}
{{ at hudson.remoting.Engine.run(Engine.java:519)}}

More agent.jar log output can be found in the attached Agent.log

On the Jenkins server side I can see the following exception in the log:

2021-03-02 13:00:31.137+0000 [id=322] WARNING o.j.r.p.i.SSLEngineFilterLayer#onRecv: [JNLP4-connect connection from jenkins.windows.node/10.119.64.14:60826]
java.lang.NullPointerException
{{ at jenkins.slaves.DefaultJnlpSlaveReceiver.afterProperties(DefaultJnlpSlaveReceiver.java:127)}}
{{ at org.jenkinsci.remoting.engine.JnlpConnectionState$2.invoke(JnlpConnectionState.java:394)}}
{{ at org.jenkinsci.remoting.engine.JnlpConnectionState.fire(JnlpConnectionState.java:337)}}
{{ at org.jenkinsci.remoting.engine.JnlpConnectionState.fireAfterProperties(JnlpConnectionState.java:391)}}
{{ at org.jenkinsci.remoting.engine.JnlpProtocol4Handler$Handler.onReceiveHeaders(JnlpProtocol4Handler.java:323)}}
{{ at org.jenkinsci.remoting.protocol.impl.ConnectionHeadersFilterLayer.onRecv(ConnectionHeadersFilterLayer.java:196)}}
{{ at org.jenkinsci.remoting.protocol.ProtocolStack$Ptr.onRecv(ProtocolStack.java:668)}}
{{ at org.jenkinsci.remoting.protocol.impl.SSLEngineFilterLayer.processRead(SSLEngineFilterLayer.java:369)}}
{{ at org.jenkinsci.remoting.protocol.impl.SSLEngineFilterLayer.onRecv(SSLEngineFilterLayer.java:117)}}
{{ at org.jenkinsci.remoting.protocol.ProtocolStack$Ptr.onRecv(ProtocolStack.java:668)}}
{{ at org.jenkinsci.remoting.protocol.NetworkLayer.onRead(NetworkLayer.java:136)}}
{{ at org.jenkinsci.remoting.protocol.impl.NIONetworkLayer.ready(NIONetworkLayer.java:160)}}
{{ at org.jenkinsci.remoting.protocol.IOHub$OnReady.run(IOHub.java:795)}}
{{ at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)}}
{{ at jenkins.security.ImpersonatingExecutorService$1.run(ImpersonatingExecutorService.java:59)}}
{{ at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)}}
{{ at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)}}
{{ at java.base/java.lang.Thread.run(Thread.java:834)}}
2021-03-02 13:00:31.137+0000 [id=322] SEVERE o.j.r.p.impl.NIONetworkLayer#ready: [JNLP4-connect connection from jenkins.windows.node/10.119.64.14:60826] Uncaught NullPointerException
java.lang.NullPointerException
{{ at jenkins.slaves.DefaultJnlpSlaveReceiver.afterProperties(DefaultJnlpSlaveReceiver.java:127)}}
{{ at org.jenkinsci.remoting.engine.JnlpConnectionState$2.invoke(JnlpConnectionState.java:394)}}
{{ at org.jenkinsci.remoting.engine.JnlpConnectionState.fire(JnlpConnectionState.java:337)}}
{{ at org.jenkinsci.remoting.engine.JnlpConnectionState.fireAfterProperties(JnlpConnectionState.java:391)}}
{{ at org.jenkinsci.remoting.engine.JnlpProtocol4Handler$Handler.onReceiveHeaders(JnlpProtocol4Handler.java:323)}}
{{ at org.jenkinsci.remoting.protocol.impl.ConnectionHeadersFilterLayer.onRecv(ConnectionHeadersFilterLayer.java:196)}}
{{ at org.jenkinsci.remoting.protocol.ProtocolStack$Ptr.onRecv(ProtocolStack.java:668)}}
{{ at org.jenkinsci.remoting.protocol.impl.SSLEngineFilterLayer.processRead(SSLEngineFilterLayer.java:369)}}
{{ at org.jenkinsci.remoting.protocol.impl.SSLEngineFilterLayer.onRecv(SSLEngineFilterLayer.java:117)}}
{{ at org.jenkinsci.remoting.protocol.ProtocolStack$Ptr.onRecv(ProtocolStack.java:668)}}
{{ at org.jenkinsci.remoting.protocol.NetworkLayer.onRead(NetworkLayer.java:136)}}
{{ at org.jenkinsci.remoting.protocol.impl.NIONetworkLayer.ready(NIONetworkLayer.java:160)}}
{{ at org.jenkinsci.remoting.protocol.IOHub$OnReady.run(IOHub.java:795)}}
{{ at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)}}
{{ at jenkins.security.ImpersonatingExecutorService$1.run(ImpersonatingExecutorService.java:59)}}
{{ at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)}}
{{ at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)}}
{{ at java.base/java.lang.Thread.run(Thread.java:834)}}

More form the Servers log output can be found in Jenkins.log and Node.log.

From the timing of the events I assume the following behavior:

1. Agent connects to server without issues
2. Server and Agent communicate a bit and then Server get a Nullpointer exception and closes the TCP channel
3. Agent tries to send data over the closed channel and sees that he is already disconnected
4. Agent tries to reconnect, but even if there was the  Nullpointer exception from 2), the server still thinks the Agent is connected and refuses the reconnect
5. Agent crashes, since it can not connect the server

So the root cause in my opinion is that there is this Nullpointer exception on Jenkins Server Side, a few seconds after an Agent connects.

I can reproduce this behavior with different Agent PCs, and even reinstalled the Jenkins Package on my Debian machine twice.

The Windows PC was connected to a different Jenkins Server (also on Debian Buster) before. There it operated without any issues. To switch to the new server, I deleted the agent.jar, the workspace and the java cache in %temp% before downloading the new agent.jar from the new server. I also updated the jre on the Windows agent during troubleshooting (from jre8 to jre11) to match the main version of the java jre.