After QB has been put behind Nginx proxy, the following error started frequently appearing in log files. Could you please shed some light on its nature?

2014-03-28 17:34:23,269 [pool-1-thread-69098] ERROR com.pmease.quickbuild.DefaultBuildEngine - Error processing build request.
 java.lang.RuntimeException: Error executing grid job.
        at com.pmease.quickbuild.grid.GridTaskFuture.get(GridTaskFuture.java:94)
        at com.pmease.quickbuild.grid.GridTaskFuture.get(GridTaskFuture.java:101)
        at com.pmease.quickbuild.DefaultBuildEngine.process(DefaultBuildEngine.java:316)
        at com.pmease.quickbuild.DefaultBuildEngine.access$000(DefaultBuildEngine.java:129)
        at com.pmease.quickbuild.DefaultBuildEngine$2.run(DefaultBuildEngine.java:1057)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
        at java.util.concurrent.FutureTask.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
 Caused by: com.pmease.quickbuild.QuickbuildException: Job no longer exists.
        at com.pmease.quickbuild.grid.GridTaskFuture.testJobs(GridTaskFuture.java:40)
        at com.pmease.quickbuild.grid.GridTaskFuture.get(GridTaskFuture.java:75)
        ... 10 more