GAE: An error occurred for the API request datasto

Question

at a simple query with one filter like this:

PersistenceManager pm = PMF.getFactory().getPersistenceManager();
Key userKey = KeyFactory.createKey(User.class.getSimpleName(), Long.parseLong(userId));

Query query = pm.newQuery(Car.class);
query.setFilter("owner == ownerParam");
query.declareParameters(Key.class.getName() + " ownerParam");
List<Car> cars = (List<Car>) query.execute(userKey);

we get following error (when over approx. 300 entries)

Caused by: com.google.apphosting.api.ApiProxy$UnknownException: An error occurred for the API request datastore_v3.Next(). at com.google.net.rpc3.client.RpcStub$RpcCallbackDispatcher$1.runInContext(RpcStub.java:784) at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:487) at com.google.tracing.TraceContext.runInContext(TraceContext.java:774) at com.google.tracing.TraceContext$DoInTraceContext.runInContext(TraceContext.java:751) at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:342) at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:334) at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:484) at com.google.net.rpc3.client.RpcStub$RpcCallbackDispatcher.rpcFinished(RpcStub.java:824) at com.google.net.rpc3.client.RpcStub$RpcCallbackDispatcher.failure(RpcStub.java:815) at com.google.net.rpc3.impl.client.RpcClientInternalContext.runCallbacks(RpcClientInternalContext.java:953) at com.google.net.rpc3.impl.client.RpcClientInternalContext.finishRpcAndNotifyApp(RpcClientInternalContext.java:856) at com.google.net.rpc3.impl.client.RpcNetChannel.afterFinishingActiveRpc(RpcNetChannel.java:1195) at com.google.net.rpc3.impl.client.RpcNetChannel.finishRpc(RpcNetChannel.java:1031) at com.google.net.rpc3.impl.client.RpcNetChannel.handleResponse(RpcNetChannel.java:2520) at com.google.net.rpc3.impl.client.RpcNetChannel.messageReceived(RpcNetChannel.java:2277) at com.google.net.rpc3.impl.client.RpcNetChannel.access$2000(RpcNetChannel.java:155) at com.google.net.rpc3.impl.client.RpcNetChannel$TransportCallback.receivedMessage(RpcNetChannel.java:3400) at com.google.net.rpc3.impl.client.RpcChannelTransportData$TransportCallback.receivedMessage(RpcChannelTransportData.java:626) at com.google.net.rpc3.impl.wire.RpcBaseTransport.receivedMessage(RpcBaseTransport.java:430) at com.google.net.eventmanager.AbstractFutureTask$Sync.innerRun(AbstractFutureTask.java:260) at com.google.net.eventmanager.AbstractFutureTask.run(AbstractFutureTask.java:121) at com.google.net.eventmanager.EventManagerImpl.runTask(EventManagerImpl.java:578) at com.google.net.eventmanager.EventManagerImpl.internalRunWorkerLoop(EventManagerImpl.java:993) at com.google.net.eventmanager.EventManagerImpl.runWorkerLoop(EventManagerImpl.java:875) at com.google.net.eventmanager.WorkerThreadInfo.runWorkerLoop(WorkerThreadInfo.java:160) at com.google.net.eventmanager.EventManagerImpl$WorkerThread.run(EventManagerImpl.java:1846)