SERVICE_UNAVAILABLE/2/no master

Elastic Stack Elasticsearch
1

hi,all
my cluster has 24 nodes 6 machines(4 nodes per machine), one server seem not work yesterday,
but my client print this error frequency:"org.elasticsearch.cluster.block.ClusterBlockException: blocked by: [SERVICE_UNAVAILABLE/2/no master]",it is confused to me,because my cluster works well.

how can ti be ?
2

supplement：
esVersion:1.7
status:green
elasticsearch.yml:
action.disable_delete_all_indices: true
bootstrap.mlockall: true

index.refresh_interval: 10s
index.number_of_shards: 32
index.number_of_replicas: 13
index.query.bool.max_clause_count: 65535
index.translog.flush_threshold_ops: 100000
index.store.throttle.type: none
index.merge.scheduler.max_thread_count: 1
index.cache.field.type: soft
indices.memory.index_buffer_size: 30%
indices.memory.min_shard_index_buffer_size: 256mb
indices.recovery.max_bytes_per_sec: 400mb
indices.fielddata.cache.size: -1

cluster.routing.allocation.cluster_concurrent_rebalance: 4
cluster.routing.allocation.node_initial_primaries_recoveries: 18

discovery.zen.ping.multicast.enabled: false
discovery.zen.ping.unicast.hosts: [XXXXXX]

discovery.zen.minimum_master_nodes: 15
gateway.recover_after_time: 5m

only when index, the problem occurs.