I have two jobs on separate machines. 
job 1 machine 1 

job 2 machine 2 

Bothe the jobs are enabled for concurrent execution. Job 1 triggers job 2 using the parametrized remote trigger plugin. Most of the time it works correctly but when there are some simultaneous requests to Job 1. THe plugin throws the following exception even though the Job 2 is triggered.

 
ERROR: Build step failed with exception
java.lang.NullPointerException
at java.io.Reader.<init>(Unknown Source)
at java.io.InputStreamReader.<init>(Unknown Source)
at org.jenkinsci.plugins.ParameterizedRemoteTrigger.RemoteBuildConfiguration.sendHTTPCall(RemoteBuildConfiguration.java:980)
at org.jenkinsci.plugins.ParameterizedRemoteTrigger.RemoteBuildConfiguration.sendHTTPCall(RemoteBuildConfiguration.java:799)
at org.jenkinsci.plugins.ParameterizedRemoteTrigger.RemoteBuildConfiguration.perform(RemoteBuildConfiguration.java:556)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:736)
at hudson.model.Build$BuildExecution.build(Build.java:206)
at hudson.model.Build$BuildExecution.doRun(Build.java:163)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:496)
at hudson.model.Run.execute(Run.java:1737)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:421)
Build step 'Trigger a remote parameterized job' marked build as failure