I'm encountering the same issue. It appears to have been introduced today in Logstash 5.6.3. I am running Logstash in a Docker container.
[2017-10-10T21:45:37,409][ERROR][logstash.outputs.elasticsearch] Encountered an unexpected error submitting a bulk request! Will retry. {:error_message=>"undefined method `sanitized' for \"http://172.17.0.1:9200/_bulk\":String", :class=>"NoMethodError", :backtrace=>["/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-7.4.0-java/lib/logstash/outputs/elasticsearch/common.rb:249:in `safe_bulk'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-7.4.0-java/lib/logstash/outputs/elasticsearch/common.rb:119:in `submit'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-7.4.0-java/lib/logstash/outputs/elasticsearch/common.rb:87:in `retrying_submit'", "/usr/share/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-7.4.0-java/lib/logstash/outputs/elasticsearch/common.rb:38:in `multi_receive'", "/usr/share/logstash/logstash-core/lib/logstash/output_delegator_strategies/shared.rb:13:in `multi_receive'", "/usr/share/logstash/logstash-core/lib/logstash/output_delegator.rb:49:in `multi_receive'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:436:in `output_batch'", "org/jruby/RubyHash.java:1342:in `each'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:435:in `output_batch'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:381:in `worker_loop'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:342:in `start_workers'"]}