Elasticsearch version:
6.3.0

Plugins installed:
s3-repository
x-pack

JVM version
1.8

OS version
Amazon Linux

Description of the problem including expected versus actual behavior:
curl -XDELETE "http://localhost:9200/_snapshot/s3-6-backup/curator-20180623143002" hangs even after a full restart of the cluster.

I've tried turning trace logging on, but it unfortunately hasn't helped.

Here is the status information on the backup:
curl -GET "http://localhost:9200/_snapshot/s3-6-backup/_status?pretty

Relevant:

{
      "snapshot" : "curator-20180623143002",
      "repository" : "s3-6-backup",
      "uuid" : "CbzLTfmbRCaGc5tu90x-9A",
      "state" : "ABORTED",
      "include_global_state" : true,
      "shards_stats" : {
        "initializing" : 0,
        "started" : 0,
        "finalizing" : 0,
        "done" : 187,
        "failed" : 320,
        "total" : 507
      },