While converting some manually made Jenkinsfiles to a code-generation framework we use to make project skeletons, I thought it is a nice idea to support parallel builds to be scheduled on one agent as well as spread over several ones. So I scripted up a generator of Jenkinsfile markup for that, and found that one running over several agents breaks every few builds in random places, with the last entry in the corresponding stage being a  java.util.TreeMap$Entry exception:

https://github.com/jimklimov/zproject/blob/jenkins-update/Jenkinsfile 

while the other one running under one agent did not break this way:

https://github.com/jimklimov/zproject/blob/jenkins-update-wip1/Jenkinsfile

One vague guess is that maybe there is a problem collecting log entries from workers in parallel? At least this exception is linked to serialization, and the scripts or programs called from the pipeline do nothing Javish...