Starting with version 2.343 Jenkins shows all nodes (agents/slaves) as offline

Those are older nodes, so still called slaves.

In jenkins-slave.err.log this is protocolled:

 
Apr. 14, 2022 11:32:02 VORM. hudson.remoting.jnlp.Main createEngine
INFORMATION: Setting up slave: Jenkins-Slave-E2
Apr. 14, 2022 11:32:02 VORM. hudson.remoting.jnlp.Main$CuiListener <init>
INFORMATION: Jenkins agent is running in headless mode.
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Locating server among http://swsbuild2.sws.local/jenkins/
Apr. 14, 2022 11:32:03 VORM. org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver resolve
INFORMATION: Remoting server accepts the following protocols: [JNLP4-connect, Ping]
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Agent discovery successful
  Agent address: **************
  Agent port:    1
  Identity:      **************
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Handshaking
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Connecting to s**************
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Trying protocol: JNLP4-connect
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Remote identity confirmed: **************
Apr. 14, 2022 11:32:03 VORM. hudson.remoting.jnlp.Main$CuiListener status
INFORMATION: Connected
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by hudson.remoting.RemoteClassLoader (file:/E:/E2/slave.jar) to method java.lang.ClassLoader.getClassLoadingLock(java.lang.String)
WARNING: Please consider reporting this to the maintainers of hudson.remoting.RemoteClassLoader
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

After reverting Jenkins to version 2.342 the issue is gone