After upgrade from 4.7.2. to version 5.18 we get this error. But it is hard to find out waht causes this error. How can we find this out? org.json.JSONException: JSONObject["actionname"] not found. at org.json.JSONObject.get(JSONObject.java:469) at org.json.JSONObject.getString(JSONObject.java:679) at com.mendix.webui.actions.client.ExecuteAction.execute(ExecuteAction.java:56) at com.mendix.webui.requesthandling.ClientRequestHandler$$anonfun$handleRequest$1.apply$mcV$sp(ClientRequestHandler.scala:300) at com.mendix.webui.requesthandling.ClientRequestHandler$$anonfun$handleRequest$1.apply(ClientRequestHandler.scala:293) at com.mendix.webui.requesthandling.ClientRequestHandler$$anonfun$handleRequest$1.apply(ClientRequestHandler.scala:293) at com.mendix.core.session.Worker$$anonfun$receive$3$$anonfun$2$$anon$1.execute(ActionDispatching.scala:126) at com.mendix.util.classloading.Runner.doRunUsingClassLoaderOf(Runner.java:32) at com.mendix.core.session.Worker$$anonfun$receive$3$$anonfun$2.apply(ActionDispatching.scala:128) at scala.util.Try$.apply(Try.scala:191) at com.mendix.core.session.Worker$$anonfun$receive$3.applyOrElse(ActionDispatching.scala:122) at akka.actor.Actor$class.aroundReceive(Actor.scala:465) at com.mendix.core.session.Worker.aroundReceive(ActionDispatching.scala:119) at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516) at akka.actor.ActorCell.invoke(ActorCell.scala:487) at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238) at akka.dispatch.Mailbox.run(Mailbox.scala:220) at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:393) at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260) at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339) at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979) at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)