monix-sample | project exemplifying client / server communications | Websocket library

I experienced the exact same error. My setting:

• Pipeline build inside a dockerized Jenkins (version 2.32.3)
• In the configuration of the job, I specified a check out into a subdirectory: Open the configuration, e.g. https://myJenkins/job/my-job/configure. At the bottom, see section Pipeline -> Additional Behaviours -> Check out into a sub-directory with Local subdirectory for repo set to, e.g., my-sub-dir.
• Expectation: Upon check out, the Jenkinsfile ends up in my-sub-dir/Jenkinsfile.
• Via the option Script path, you configure the location of the Jenkinsfile so that Jenkins can start the build. I put my-sub-dir/Jenkinsfile as value.

I then received the exception you pasted in your question. I fixed it by setting Script Path to Jenkinsfile. If you don't specify a sub-directory for check out, then still try double checking values for Script Path.

Note: I have another Jenkins instance at work. There I have to specify Script Path including the custom check out sub-directory (as mentioned in Expectation above).