Best way to debug DeadLetterQueue crashing

[smbullet]

What's the best way to debug something like this?

[2018-03-26T11:30:57,698][ERROR][logstash.pipeline        ] A plugin had an unrecoverable error. Will restart this plugin.
  Plugin: <LogStash::Inputs::DeadLetterQueue path=>"/home/logstash_user/logstash/data/dead_letter_queue", add_field=>{"original"=>"true"}, id=>"1e7a92ea233ca8d71d4b3d2211fc810e72ab1222-19", enable_metric=>true, codec=><LogStash::Codecs::Plain id=>"plain_82027229-147b-43f7-87fb-7a7e7acb75f6", enable_metric=>true, charset=>"UTF-8">, pipeline_id=>"main", commit_offsets=>true>
  Error:
[2018-03-26T11:30:58,698][ERROR][logstash.pipeline        ] A plugin had an unrecoverable error. Will restart this plugin.
  Plugin: <LogStash::Inputs::DeadLetterQueue path=>"/home/logstash_user/logstash/data/dead_letter_queue", add_field=>{"original"=>"true"}, id=>"1e7a92ea233ca8d71d4b3d2211fc810e72ab1222-19", enable_metric=>true, codec=><LogStash::Codecs::Plain id=>"plain_82027229-147b-43f7-87fb-7a7e7acb75f6", enable_metric=>true, charset=>"UTF-8">, pipeline_id=>"main", commit_offsets=>true>
  Error:
[2018-03-26T11:30:59,699][ERROR][logstash.pipeline        ] A plugin had an unrecoverable error. Will restart this plugin.
  Plugin: <LogStash::Inputs::DeadLetterQueue path=>"/home/logstash_user/logstash/data/dead_letter_queue", add_field=>{"original"=>"true"}, id=>"1e7a92ea233ca8d71d4b3d2211fc810e72ab1222-19", enable_metric=>true, codec=><LogStash::Codecs::Plain id=>"plain_82027229-147b-43f7-87fb-7a7e7acb75f6", enable_metric=>true, charset=>"UTF-8">, pipeline_id=>"main", commit_offsets=>true>
  Error:
[2018-03-26T11:31:00,701][ERROR][logstash.pipeline        ] A plugin had an unrecoverable error. Will restart this plugin.
  Plugin: <LogStash::Inputs::DeadLetterQueue path=>"/home/logstash_user/logstash/data/dead_letter_queue", add_field=>{"original"=>"true"}, id=>"1e7a92ea233ca8d71d4b3d2211fc810e72ab1222-19", enable_metric=>true, codec=><LogStash::Codecs::Plain id=>"plain_82027229-147b-43f7-87fb-7a7e7acb75f6", enable_metric=>true, charset=>"UTF-8">, pipeline_id=>"main", commit_offsets=>true>
  Error:

As you can see, the error message isn't very descriptive. This continuously occurs every second indefinitely. Any suggestions?

EDIT: I stumbled upon this: https://github.com/elastic/logstash/issues/7820
Seems to be the same issue I'm having. I'm running 5.5.0 so I'll try upgrading to 6.2.3 and see if it fixes itself.