Elasticsearch cluster: java.lang.IllegalStateException: Future got interrupted - elastic-stack

Question

After a restart, our Elasticsearch cluster is showing this in the logs:
[2018-08-30T14:13:15,111][ERROR][o.e.x.m.c.c.ClusterStatsCollector] [myelastic01] collector [cluster_stats] failed to collect data
java.lang.IllegalStateException: Future got interrupted
at org.elasticsearch.common.util.concurrent.FutureUtils.get(FutureUtils.java:53) ~[elasticsearch-6.3.2.jar:6.3.2]
at org.elasticsearch.action.support.AdapterActionFuture.actionGet(AdapterActionFuture.java:34) ~[elasticsearch-6.3.2.jar:6.3.2]
(...)
Caused by: java.lang.InterruptedException
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:998) ~[?:1.8.0_181]
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304) ~[?:1.8.0_181]
(...)
[2018-08-30T14:15:22,972][ERROR][o.e.x.m.c.i.IndexStatsCollector] [myelastic01] collector [index-stats] failed to collect data
org.elasticsearch.cluster.block.ClusterBlockException: blocked by: [SERVICE_UNAVAILABLE/1/state not recovered / initialized];
at org.elasticsearch.cluster.block.ClusterBlocks.globalBlockedException(ClusterBlocks.java:166) ~[elasticsearch-6.3.2.jar:6.3.2]
at org.elasticsearch.action.admin.indices.stats.TransportIndicesStatsAction.checkGlobalBlock(TransportIndicesStatsAction.java:68) ~[elasticsearch-6.3.2.jar:6.3.2]
at org.elasticsearch.action.admin.indices.stats.TransportIndicesStatsAction.checkGlobalBlock(TransportIndicesStatsAction.java:45) ~[elasticsearch-6.3.2.jar:6.3.2]
at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction.<init>(TransportBroadcastByNodeAction.java:255) ~[elasticsearch-6.3.2.j
ar:6.3.2]
[2018-08-30T14:18:50,307][WARN ][r.suppressed ] path: /.kibana/_search, params: {ignore_unavailable=true, index=.kibana, filter_path=aggregations.types.buckets}
org.elasticsearch.action.search.SearchPhaseExecutionException: all shards failed
at org.elasticsearch.action.search.AbstractSearchAsyncAction.onPhaseFailure(AbstractSearchAsyncAction.java:288) ~[elasticsearch-6.3.2.jar:6.3.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.executeNextPhase(AbstractSearchAsyncAction.java:128) ~[elasticsearch-6.3.2.jar:6.3.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.onPhaseDone(AbstractSearchAsyncAction.java:249) ~[elasticsearch-6.3.2.jar:6.3.2]
(...)
[2018-08-30T14:18:50,304][WARN ][r.suppressed ] path: /.kibana/doc/config%3A6.3.2, params: {index=.kibana, id=config:6.3.2, type=doc}
org.elasticsearch.action.NoShardAvailableActionException: No shard available for [get [.kibana][doc][config:6.3.2]: routing [null]]
at org.elasticsearch.action.support.single.shard.TransportSingleShardAction$AsyncSingleAction.perform(TransportSingleShardAction.java:207) ~[elasticsearch-6.3.2.jar:
6.3.2]
at org.elasticsearch.action.support.single.shard.TransportSingleShardAction$AsyncSingleAction.start(TransportSingleShardAction.java:186) ~[elasticsearch-6.3.2.jar:6.
3.2]
at org.elasticsearch.action.support.single.shard.TransportSingleShardAction.doExecute(TransportSingleShardAction.java:95) ~[elasticsearch-6.3.2.jar:6.3.2]
(...)
This is a ELK stack which was working before, the configuration has not been modified. What could be the cause? I've searched around but found nothing relevant.