Greetings,
We are noticing this error frequently in Jenkins v2.426.3. The build fails with a ClassNotFoundException error message that doesn’t make sense because the missing class is a var that exists in the DSL Lib. for example:

14:15:09  Also:   org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: c424dd38-fa50-448c-a0c7-a697d0e81c7a
14:15:09  java.lang.ClassNotFoundException: archiveCorefiles
14:15:09      at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:445)

You will probably also see these similar error just above it in the console log:

14:15:07  Error when executing always post condition:
14:15:07  Also:   org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: 54c465d8-3c65-48a9-9995-12fde0a1ad86
14:15:07  java.lang.ClassNotFoundException: postStatusHistoryAndResults

…

14:15:09  Error when executing cleanup post condition:
14:15:09  Also:   org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: d7235587-a635-4b64-9cbf-dec13ec12a83
14:15:09  java.lang.ClassNotFoundException: cleanupBuildAgentWorkspace

Workaround being followed :
Log into jenkins server
cd to home/workspaces
rm -rf <job's workspace>

Note: This should be done when a job isn't actively running

Please find attached error log for full text of the mentioned error. 

Could you please help regarding this issue. Thank you and appreciate your help.