KryoSerializer cannot find my SparkKryoRegistrator

Question

I am using Spark 2.0.2 on Amazon emr-5.2.1 in client mode. I use Kryo serialisation and register our classes in our own KryoRegistrator:

val sparkConf = new SparkConf()
    .set("spark.serializer", "org.apache.spark.serializer.KryoSerializer")
    .set("spark.kryo.registrator", classOf[de.gaf.ric.workflow.RicKryoRegistrator].getName)
    .set("spark.kryo.registrationRequired", "true")
    .set("spark.kryoserializer.buffer.max", "512m")
implicit val sc = new SparkContext(sparkConf)

The process starts fine, but after some minutes, I get the following exception on the executors:

17/02/02 16:22:34 ERROR RetryingBlockFetcher: Failed to fetch block rdd_3641_12, and will not retry (0 retries)
java.lang.RuntimeException: org.apache.spark.SparkException: Failed to register classes with Kryo
    at org.apache.spark.serializer.KryoSerializer.newKryo(KryoSerializer.scala:129)
    at org.apache.spark.serializer.KryoSerializerInstance.borrowKryo(KryoSerializer.scala:274)
    at org.apache.spark.serializer.KryoSerializerInstance.<init>(KryoSerializer.scala:259)
    at org.apache.spark.serializer.KryoSerializer.newInstance(KryoSerializer.scala:175)
    at org.apache.spark.serializer.SerializerManager.dataSerializeWithExplicitClassTag(SerializerManager.scala:141)
    at org.apache.spark.storage.BlockManager.org$apache$spark$storage$BlockManager$$doGetLocalBytes(BlockManager.scala:499)
    at org.apache.spark.storage.BlockManager$$anonfun$getLocalBytes$2.apply(BlockManager.scala:474)
    at org.apache.spark.storage.BlockManager$$anonfun$getLocalBytes$2.apply(BlockManager.scala:474)
    at scala.Option.map(Option.scala:146)
    at org.apache.spark.storage.BlockManager.getLocalBytes(BlockManager.scala:474)
    at org.apache.spark.storage.BlockManager.getBlockData(BlockManager.scala:280)
    at org.apache.spark.network.netty.NettyBlockRpcServer$$anonfun$2.apply(NettyBlockRpcServer.scala:60)
    at org.apache.spark.network.netty.NettyBlockRpcServer$$anonfun$2.apply(NettyBlockRpcServer.scala:60)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
    at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
    at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:186)
    at org.apache.spark.network.netty.NettyBlockRpcServer.receive(NettyBlockRpcServer.scala:60)
    at org.apache.spark.network.server.TransportRequestHandler.processRpcRequest(TransportRequestHandler.java:159)
    at org.apache.spark.network.server.TransportRequestHandler.handle(TransportRequestHandler.java:107)
    at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:119)
    at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:51)
    at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:266)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at org.apache.spark.network.util.TransportFrameDecoder.channelRead(TransportFrameDecoder.java:85)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:846)
    at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
    at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: de.gaf.ric.workflow.RicKryoRegistrator
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at org.apache.spark.serializer.KryoSerializer$$anonfun$newKryo$5.apply(KryoSerializer.scala:124)
    at org.apache.spark.serializer.KryoSerializer$$anonfun$newKryo$5.apply(KryoSerializer.scala:124)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
    at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
    at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:186)
    at org.apache.spark.serializer.KryoSerializer.newKryo(KryoSerializer.scala:124)
    ... 43 more

    at org.apache.spark.network.client.TransportResponseHandler.handle(TransportResponseHandler.java:189)
    at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:121)
    at org.apache.spark.network.server.TransportChannelHandler.channelRead0(TransportChannelHandler.java:51)
    at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:266)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at org.apache.spark.network.util.TransportFrameDecoder.channelRead(TransportFrameDecoder.java:85)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:308)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:294)
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:846)
    at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
    at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)
    at java.lang.Thread.run(Thread.java:745)

The class RicKryoRegistrator is definitively included in my uber JAR, I double checked that. It is also transferred to the executors:

17/02/02 16:19:02 INFO Executor: Fetching spark://172.31.20.106:41032/jars/app-imageprocessing-0.13.0-20170202.112920-1.jar with timestamp 1486048690879
17/02/02 16:19:02 INFO TransportClientFactory: Successfully created connection to /172.31.20.106:41032 after 23 ms (0 ms spent in bootstraps)
17/02/02 16:19:02 INFO Utils: Fetching spark://172.31.20.106:41032/jars/app-imageprocessing-0.13.0-20170202.112920-1.jar to /mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/spark-8172edd9-d1c7-40c9-ad9b-74b2bd9dbad9/fetchFileTemp6474512860106916303.tmp
17/02/02 16:19:03 INFO Utils: Copying /mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/spark-8172edd9-d1c7-40c9-ad9b-74b2bd9dbad9/-68603321486048690879_cache to /mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/container_1486039395474_0012_01_000011/./app-imageprocessing-0.13.0-20170202.112920-1.jar
17/02/02 16:19:03 INFO Executor: Adding file:/mnt/yarn/usercache/hadoop/appcache/application_1486039395474_0012/container_1486039395474_0012_01_000011/./app-imageprocessing-0.13.0-20170202.112920-1.jar to class loader

Reading the source code of org.apache.spark.serializer.KryoSerializer, I see that it uses the following ClassLoader:

val classLoader = defaultClassLoader.getOrElse(Thread.currentThread.getContextClassLoader)

Could it be that the defaultClassLoader is not set and that my uber JAR is not included in the Thread.currentThread.getContextClassLoader? What else could be the reason?