java.io.IOException: Message: Remote call on jenkins-freebsd12-worker failed; Cause: java.lang.UnsupportedClassVersionError: Failed to load org.jenkinsci.plugins.gitclient.verifier.AbstractJGitHostKeyVerifier; toString: java.io.IOException: Remote call on jenkins-freebsd12-worker failed

java.io.IOException: Message: Remote call on JNLP4-connect connection from swarmclient:12345 failed; Cause: java.lang.UnsupportedClassVersionError: Failed to load org.jenkinsci.plugins.gitclient.verifier.AbstractJGitHostKeyVerifier; toString: java.io.IOException: Remote call on JNLP4-connect connection from swarmclient:12345 

I am not quickly sure which version of Java is on "swarmclient" above (OS is openbsd-6.4), but for the FreeBSD 12 worker it was Java 8:

$ java -version
openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)
OpenJDK 64-Bit Server VM (build 25.292-b10, mixed mode) 

Git-client-plugin 3.11.1 worked there, so I suppose a dependency (for org.jenkinsci.plugins.gitclient.verifier.AbstractJGitHostKeyVerifier?) snuck in as a JAR build for newer version? Rebuilding the plugin itself with JDK8 as a quick shot did not help (UPDATE: Thinking of it, I just changed JAVA_HOME and did not mvn clean the build area; retrying). I'll know more later (I merely commute now).

If Java 8 is not forbidden yet, just "not recommended anymore", this is a critical issue generally.

In particular, that CI farm is using Jenkins LTS (2.346.2) with agents for some older build environments for legacy appliances running the Java 8 as newest available there and then. So the breakage was not quite expected.