We encountered a similar issue with version 2.7. It was only one incident out of hundreds of runs, but might as well keep this ticket open if it can still happen.
[Pipeline] Start of Pipeline
[Pipeline] lock
Trying to acquire lock on [rcprod]
Resource [rcprod] did not exist. Created.
[Pipeline] // lock
[Pipeline] End of Pipeline
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
at java.util.ArrayList$Itr.next(ArrayList.java:859)
at org.jenkins.plugins.lockableresources.LockableResourcesManager.fromName(LockableResourcesManager.java:182)
at org.jenkins.plugins.lockableresources.queue.LockableResourcesStruct.<init>(LockableResourcesStruct.java:83)
at org.jenkins.plugins.lockableresources.LockStepExecution.start(LockStepExecution.java:56)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeStep(DSL.java:286)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:179)
...