This happens in test with module-based shard, tell-based protocol. A listener is registered (on a member which is not leader). There is a transaction writer on each member, each finishes successfully [0]. The unscubscribe-dtcl call fails [1]:
Problem accessing /restconf/operations/odl-mdsal-lowlevel-control:unsubscribe-dtcl. Reason:
<pre> Server Error</pre></p><h3>Caused by:</h3><pre>java.lang.NullPointerException
at org.opendaylight.controller.clustering.it.provider.impl.IdIntsListener.checkEqual(IdIntsListener.java:57)
at org.opendaylight.controller.clustering.it.provider.MdsalLowLevelTestProvider.unsubscribeDtcl(MdsalLowLevelTestProvider.java:480)

If the listener got zero notifications, the unsubscribe-dtcl should report that in a readable message. The clustering bug is in the fact the DTC notifications were lost (or the NPE happened for some other reason).

Be the way, the UnreachableMember seems to happen in every test with 3 writers/producers (with half million integer array created in the initial transaction).

[0] https://logs.opendaylight.org/releng/jenkins092/controller-csit-3node-clustering-only-carbon/715/archives/log.html.gz#s1-s36-t3-k2-k15
[1] https://logs.opendaylight.org/releng/jenkins092/controller-csit-3node-clustering-only-carbon/715/archives/log.html.gz#s1-s36-t3-k2-k17-k2-k1-k4-k7-k1