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How to solve generic 'Unexpected error in launching an agent. This is probably a bug in Jenkins' error on Jenkins agent startup

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I have configured a docker cloud dynamic agent creation with the following settings: and the agent config as follows:

but they fail to start. Been googling around but since I have little to nothing to go on with the logs, I don't know how to fix this.

Connecting to docker container 2169d7b0f9de955c89916da421dc6e04f41104423d0fdcd28796162225cf491f, running command java -jar //remoting-4.11.2.jar -noReconnect -noKeepAlive -agentLog //agent.log
HTTP/1.1 101 UPGRADED
Content-Type: application/vnd.docker.raw-stream
Connection: Upgrade
Upgrade: tcp
Api-Version: 1.41
Docker-Experimental: false
Ostype: linux
Server: Docker/20.10.12 (linux)
ERROR: Unexpected error in launching an agent. This is probably a bug in Jenkins
Also:   java.lang.Throwable: launched here
    at hudson.slaves.SlaveComputer._connect(SlaveComputer.java:282)
    at hudson.model.Computer.connect(Computer.java:440)
    at hudson.slaves.SlaveComputer.doLaunchSlaveAgent(SlaveComputer.java:796)
    at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627)
    at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:393)
    at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:405)
    at org.kohsuke.stapler.interceptor.RequirePOST$Processor.invoke(RequirePOST.java:77)
    at org.kohsuke.stapler.PreInvokeInterceptedFunction.invoke(PreInvokeInterceptedFunction.java:26)
    at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:208)
    at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:141)
    at org.kohsuke.stapler.MetaClass$11.doDispatch(MetaClass.java:536)
    at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
The last part of the logs stay:
java.io.EOFException: unexpected stream termination
    at hudson.remoting.ChannelBuilder.negotiate(ChannelBuilder.java:464)
    at hudson.remoting.ChannelBuilder.build(ChannelBuilder.java:409)
    at hudson.slaves.SlaveComputer.setChannel(SlaveComputer.java:432)
    at hudson.slaves.SlaveComputer.setChannel(SlaveComputer.java:399)
    at io.jenkins.docker.connector.DockerComputerAttachConnector$DockerAttachLauncher.launch(DockerComputerAttachConnector.java:321)
    at hudson.slaves.DelegatingComputerLauncher.launch(DelegatingComputerLauncher.java:63)
    at io.jenkins.docker.connector.DockerDelegatingComputerLauncher.launch(DockerDelegatingComputerLauncher.java:37)
    at hudson.slaves.SlaveComputer.lambda$_connect$0(SlaveComputer.java:293)
    at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
    at jenkins.security.ImpersonatingExecutorService$2.call(ImpersonatingExecutorService.java:80)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
and in jenkins logs I see:
96a29780b14e340c1667f2 for node Jenkins JDK11 agent-00000a8utz442 from image: jenkins/agent:alpine
2022-01-27 22:56:32.628+0000 [id=188]   INFO    i.j.d.c.DockerMultiplexedInputStream#readInternal: stderr from Jenkins JDK11 agent-00000a8utz442 (4611c8568d1b73e6a8b7e54613f7ef06c79210f03a96a29780b14e340c1667f2): Jan 27, 2022 10:56:32 PM org.jenkinsci.remoting.engine.WorkDirManager setupLogging
INFO: Using /agent.log as an agent error log destination; output log will not be generated
2022-01-27 22:56:32.628+0000 [id=188]   INFO    i.j.d.c.DockerMultiplexedInputStream#readInternal: stderr from Jenkins JDK11 agent-00000a8utz442 (4611c8568d1b73e6a8b7e54613f7ef06c79210f03a96a29780b14e340c1667f2): Exception in thread "main"
2022-01-27 22:56:32.629+0000 [id=188]   INFO    i.j.d.c.DockerMultiplexedInputStream#readInternal: stderr from Jenkins JDK11 agent-00000a8utz442 (4611c8568d1b73e6a8b7e54613f7ef06c79210f03a96a29780b14e340c1667f2): java.io.FileNotFoundException: /agent.log (Permission denied)
2022-01-27 22:56:32.629+0000 [id=188]   INFO    i.j.d.c.DockerMultiplexedInputStream#readInternal: stderr from Jenkins JDK11 agent-00000a8utz442 (4611c8568d1b73e6a8b7e54613f7ef06c79210f03a96a29780b14e340c1667f2):    at java.base/java.io.FileOutputStream.open0(Native Method)
2022-01-27 22:56:32.629+0000 [id=188]   INFO    i.j.d.c.DockerMultiplexedInputStream#readInternal: stderr from Jenkins JDK11 agent-00000a8utz442 (4611c8568d1b73e6a8b7e54613f7ef06c79210f03a96a29780b14e340c1667f2):    at java.base/java.io.FileOutputStream.open(Unknown Source)
2022-01-27 22:56:32.629+0000 [id=188]   INFO    i.j.d.c.DockerMultiplexedInputStream#readInternal: stderr from Jenkins JDK11 agent-00000a8utz442 (4611c8568d1b73e6a8b7e54613f7ef06c79210f03a96a29780b14e340c1667f2):    at java.base/java.io.FileOutputStream.<init>(Unknown Source)
Which is actually not that much info either.
                Using /agent.log as an agent error log destination followed by /agent.log (Permission denied) makes perfect sense.  Jenkins is trying to write the log file to the root directory, which obviously won't work.  Figure out where in the config the log file is defined and fix the path to somewhere your process can write.
– Jim Garrison
                Jan 28, 2022 at 1:27
                The fact the launch command shows -jar //remoting-4.11.2.jar, the double slash suggests a variable (Remote FS Root ?) has not been defined. Hmm (checks image) ....
– Ian W
                Jan 28, 2022 at 5:32
                Yes, you are both correct, the root was ill defined. Updated it and is working as expected. Thanks!
– Pedro Zuppelli
                Feb 2, 2022 at 15:01
                @PedroZuppelli can you please guide me with the steps which needs to be followed to get rid of this. It will be really helpful for me.
– Arpit
                Feb 5, 2022 at 12:07
                as the answer states, the Remote File System Root in the agent configuration was missing, updating it solves the issue
– Pedro Zuppelli
                Feb 10, 2022 at 14:22
I saw a similar error with an existing setup that used to work but started failing suddenly (probably after something auto updated).
In my case inspecting the system logs in Dashboard -> Manage Jenkins -> System Log was more enlightening. It turned out that my Jenkins agent was outdated:
stderr from docker-0006huns8vq4o
(60811a53ee637a5e76eb7390f00481945663120067f89b3d08323344b8c93f42):
java.lang.UnsupportedClassVersionError: hudson/remoting/Launcher has
been compiled by a more recent version of the Java Runtime (class file
version 55.0), this version of the Java Runtime only recognizes class
file versions up to 52.0
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