Hello,
I looked over you logs. I didn’t notice any “connection” issues, mainly bulk request issues.
ERROR: org.graylog2.indexer.messages.Messages - Couldn't bulk index 1000 messages.
What is your Heap for Elasticsearch?
ERROR: org.graylog2.outputs.BlockingBatchedESOutput - Unable to flush message buffer
Maybe the answer can be found here.
Insure elasticsearch is working correctly.
Perhaps this site might help