The following example uses an ExecutionContext with direct, synchronous execution.

  def futureTest: Int = {
    import scala.concurrent.duration._
    implicit val immediateContext = new scala.concurrent.ExecutionContext {
      override def execute(runnable: Runnable) { runnable.run() }
      override def reportFailure(t: Throwable) { t.printStackTrace() }
    }
    val normal = scala.concurrent.ExecutionContext.fromExecutor(java.util.concurrent.Executors.newCachedThreadPool())
    val initial = scala.concurrent.Future({ Thread.sleep(1000); 1 })(normal)
    val res = initial.flatMap(x1 => scala.concurrent.Future.successful(2)).flatMap(x2 => {
      val res2 = scala.concurrent.Future.successful(3).flatMap(x3 => scala.concurrent.Future.successful(4))
      scala.concurrent.Await.ready(res2, 300 seconds)   // this throws the IllegalArgumentException
    })
    scala.concurrent.Await.result(res, 300 seconds)
  }
  futureTest

It fails with

java.lang.IllegalArgumentException: requirement failed
at scala.Predef$.require(Predef.scala:221)
at scala.concurrent.Future$InternalCallbackExecutor$Batch.run(Future.scala:628)
at scala.concurrent.Future$InternalCallbackExecutor$.scala$concurrent$Future$InternalCallbackExecutor$$unbatchedExecute(Future.scala:691)
at scala.concurrent.Future$InternalCallbackExecutor$Batch.blockOn(Future.scala:669)
at scala.concurrent.Await$.ready(package.scala:86)
at $anonfun$3.apply(:17)
at $anonfun$3.apply(:15)
at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:251)
at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:249)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
at $anon$1.execute(:10)
at scala.concurrent.impl.CallbackRunnable.executeWithValue(Promise.scala:40)
at scala.concurrent.impl.Promise$DefaultPromise.tryComplete(Promise.scala:248)
at scala.concurrent.Promise$class.complete(Promise.scala:55)
at scala.concurrent.impl.Promise$DefaultPromise.complete(Promise.scala:153)
at scala.concurrent.Future$$anonfun$flatMap$1$$anonfun$apply$3.apply(Future.scala:254)
at scala.concurrent.Future$$anonfun$flatMap$1$$anonfun$apply$3.apply(Future.scala:254)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.processBatch$1(Future.scala:640)
at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.apply$mcV$sp(Future.scala:655)
at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.apply(Future.scala:632)
at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.apply(Future.scala:632)
at scala.concurrent.BlockContext$.withBlockContext(BlockContext.scala:72)
at scala.concurrent.Future$InternalCallbackExecutor$Batch.run(Future.scala:631)
at scala.concurrent.Future$InternalCallbackExecutor$.scala$concurrent$Future$InternalCallbackExecutor$$unbatchedExecute(Future.scala:691)
at scala.concurrent.Future$InternalCallbackExecutor$.execute(Future.scala:682)
at scala.concurrent.impl.CallbackRunnable.executeWithValue(Promise.scala:40)
at scala.concurrent.impl.Promise$KeptPromise.onComplete(Promise.scala:333)
at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:254)
at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:249)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
at $anon$1.execute(:10)
at scala.concurrent.impl.CallbackRunnable.executeWithValue(Promise.scala:40)
at scala.concurrent.impl.Promise$DefaultPromise.tryComplete(Promise.scala:248)
at scala.concurrent.Promise$class.complete(Promise.scala:55)
at scala.concurrent.impl.Promise$DefaultPromise.complete(Promise.scala:153)
at scala.concurrent.impl.Future$PromiseCompletingRunnable.run(Future.scala:23)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:744)

The stack trace is from 2.10.3, but it also fails in 2.11.0-RC1. The stack strace looks very similar to that of #8069.
Batch#blockOn looks suspicious, setting _tasksLocal to Nil and then calling run which requires it to be null. Also the "WARNING" comment from BatchingExecutor seems to apply to InternalCallbackExecutor.