I got this ConcurrentModificationException when using "lock" step for several pipelines running on the same master at the same time. I believe it can happen when new locks are being created while the list of locks is being looped through.

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:156)
at org.jenkins.plugins.lockableresources.queue.LockableResourcesStruct.<init>(LockableResourcesStruct.java:83)
at org.jenkins.plugins.lockableresources.LockStepExecution.start(LockStepExecution.java:62)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeStep(DSL.java:268)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:176)
at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:122)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:48)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.methodCall(DefaultInvoker.java:20)