carbon: IllegalStateException: Can't create ProxyReadTransaction

It happenes when when accessing netconf-test-device's mount point.
It is present in the last few jobs https://jenkins.opendaylight.org/releng/view/netconf/job/netconf-csit-3node-clustering-all-carbon/.

E.g.
(#328) https://logs.opendaylight.org/releng/jenkins092/netconf-csit-3node-clustering-all-carbon/328/log.html.gz#s1-s3-t8-k2-k1-k2-k1-k4-k1
(#330) https://logs.opendaylight.org/releng/jenkins092/netconf-csit-3node-clustering-all-carbon/330/log.html.gz#s1-s3-t10-k2-k1-k2-k1-k4-k1
(#331) https://logs.opendaylight.org/releng/jenkins092/netconf-csit-3node-clustering-all-carbon/331/log.html.gz#s1-s8-t10-k2-k1-k2-k1-k4-k1

The netconf device is configured on node1 and the failures may happen probably on any node, because it happened on node1 (#328) and node3 (#330,#331).

Even the caused by exception says in #331
Caused by: akka.pattern.AskTimeoutException: Recipient[Actor[akka://opendaylight-cluster-data/user/akka.tcp:opendaylight-cluster-data@10.29.13.100:2550_netconf-test-device#-917568787]] had already been terminated. Sender[null] sent the message of type "org.opendaylight.netconf.topology.singleton.messages.transactions.NewReadTransactionRequest".

and in jobs #330, #328
Caused by: java.util.concurrent.TimeoutException: Futures timed out after [5 seconds]
at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:223)
at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:227)
at scala.concurrent.Await$$anonfun$result$1.apply(package.scala:190)
at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:53)
at scala.concurrent.Await$.result(package.scala:190)
at scala.concurrent.Await.result(package.scala)
at org.opendaylight.netconf.topology.singleton.impl.ProxyDOMDataBroker.newReadOnlyTransaction(ProxyDOMDataBroker.java:68)

no akka problems (such as unreachable nodes) are present in the log files.