ConcurrentModificationException

I get a ConcurrentModificationException in the log. It happens when importing master data (each object becomes a webservice call, so many calls in a short period of time). It happens approximately once every hour and lasts for 15 minutes. During that time, messages are handled, but after 15 minutes it starts working fine again (until 1 hour later). What can be the cause of this?

Aug 11 14:18:46.751 127.0.0.1 tr10000: ERROR - ActorSystem: Unknown message: Failure(java.util.ConcurrentModificationException)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (1/13) com.mendix.systemwideinterfaces.MendixRuntimeException: Unknown message: Failure(java.util.ConcurrentModificationException)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (2/13) at com.mendix.core.session.ActionDispatcher$$anonfun$receive$2.applyOrElse(ActionDispatching.scala:132)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (3/13) at akka.actor.Actor$class.aroundReceive(Actor.scala:465)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (4/13) at com.mendix.core.session.ActionDispatcher.aroundReceive(ActionDispatching.scala:111)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (5/13) at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (6/13) at akka.actor.ActorCell.invoke(ActorCell.scala:487)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (7/13) at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (8/13) at akka.dispatch.Mailbox.run(Mailbox.scala:220)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (9/13) at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:393)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (10/13)            at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (11/13)            at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (12/13)            at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
Aug 11 14:18:46.757 127.0.0.1 tr10000: ERROR - ActorSystem: (13/13)            at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)