com.google.appengine.api.modules.ModulesException: Unknown error: '4'

[Michael Osofsky via StackOverflow]

I'm getting the following exception on Google App Engine. It's intermittent as it only occurred twice in ~400 calls to the same code.

14:56:46.118 /insightTimeseriesDownloader
com.google.appengine.api.modules.ModulesException: Unknown error: '4'
    at com.google.appengine.api.modules.ModulesServiceImpl$ModulesServiceFutureWrapper.convertApplicationException(ModulesServiceImpl.java:382)
    at com.google.appengine.api.modules.ModulesServiceImpl$ModulesServiceFutureWrapper.convertException(ModulesServiceImpl.java:352)
    at com.google.appengine.api.utils.FutureWrapper.get(FutureWrapper.java:96)
    at com.google.appengine.api.modules.ModulesServiceImpl.getAsyncResult(ModulesServiceImpl.java:104)
    at com.google.appengine.api.modules.ModulesServiceImpl.getVersionHostname(ModulesServiceImpl.java:321)
    at com.netbase.server.TaskManager.enqueueNextTask(TaskManager.java:37)
    at com.netbase.server.TaskManager.processTask(TaskManager.java:90)
    at com.netbase.server.TaskManagerServlet.doHelper(TaskManagerServlet.java:68)
    at com.netbase.server.TaskManagerServlet.doPost(TaskManagerServlet.java:33)

This occurred on 2014-12-02 at both 14:56:46.118 and 15:54:45.547 but executed successfully many times. So I'm wondering if Google App Engine was going through maintenance or experiencing some kind of glitch. Does Google App Engine have a web page showing its uptime and a log of service problems?

Here is the code that's throwing the exception:

public static void enqueueNextTask (String topicId, Date requestDate, Key<InsightDataSet> parentKey) {
    // Add the task to the default queue
    Queue queue = QueueFactory.getDefaultQueue();
    TaskOptions taskOptions = TaskOptions.Builder.withUrl("/insightTimeseriesDownloader").
            param("topicId", topicId).
            param("requestDate", MySharedUtil.dateToTimestamp(requestDate)).
            header("Host", ModulesServiceFactory.getModulesService().getVersionHostname("nbsocialmetrics-backend",
                    ModulesServiceFactory.getModulesService().getCurrentVersion())).
            param("parentKey", parentKey.getString());
    queue.add(taskOptions);     
}

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http://stackoverflow.com/questions/27279009/com-google-appengine-api-modules-modulesexception-unknown-error-4