This is continuation of CONTROLLER-1731 (from comment 3 on) with better title and less old information.

The test scenario: Each member starts transaction producer. Leader is isolated for a time slightly longer than 120 seconds. New leader is elected, tests sees the isolated produred report RequestTimeoutException, at that point the preoducer unregisters, while the member is still isolated.
After rejoin, the test attempts to start new producer. During producer registration, frontend rejects it, as the new leader still believes the old producer is registered.

The complete karaf.log is here [10], the final relevant segment:
2017-07-11 11:58:31,266 | DEBUG | qtp138207402-77 | ShardedDOMDataTree | 147 - org.opendaylight.mdsal.dom-broker - 2.2.1.SNAPSHOT | Creating producer org.opendaylight.mdsal.dom.broker.ShardedDOMDataTreeProducer@e86db
2017-07-11 11:58:31,266 | TRACE | qtp138207402-77 | ShardedDOMDataTree | 147 - org.opendaylight.mdsal.dom-broker - 2.2.1.SNAPSHOT | Adding subtree DOMDataTreeIdentifier{datastore=CONFIGURATION, root=/(tag:opendaylight.org
,2017:controller:yang:lowlevel:target?revision=2017-02-15)id-ints/id-int/id-int[

{(tag:opendaylight.org,2017:controller:yang:lowlevel:target?revision=2017-02-15)id=prefix-3}

]} to map.
2017-07-11 11:58:31,291 | ERROR | qtp138207402-77 | ContainerResponse | 68 - com.sun.jersey.jersey-server - 1.17.0 | The RuntimeException could not be mapped to a response, re-throwing to the HTTP container
org.opendaylight.controller.cluster.datastore.exceptions.TimeoutException: Sending message class org.opendaylight.controller.cluster.sharding.messages.ProducerCreated to actor Actorakka://opendaylight-cluster-data/user/ShardedDOMDataTreeFrontend#725503709 failed. Try again later.
at org.opendaylight.controller.cluster.datastore.utils.ActorContext.executeOperation(ActorContext.java:318)
at org.opendaylight.controller.cluster.sharding.DistributedShardedDOMDataTree.createProducer(DistributedShardedDOMDataTree.java:305)
at Proxy547e830a_606c_4629_8e3c_527344f37367.createProducer(Unknown Source)
at Proxya432b2a8_a907_437a_a7a2_58deacd6d56e.createProducer(Unknown Source)
at org.opendaylight.controller.clustering.it.provider.impl.ProduceTransactionsHandler.start(ProduceTransactionsHandler.java:67)
...

[10] https://logs.opendaylight.org/sandbox/jenkins091/controller-csit-3node-clustering-li-only-carbon/4/odl3_karaf.log.gz