Investigating reindexing error (1.7 -> 5.6)


I am trying to reindex my 1.7.5 ES cluster into a 5.6 one using Reindex API. While most indices are migrated successfully, for one of them I am getting the following error:

org.elasticsearch.ElasticsearchException: Error parsing the response, remote is likely not an Elasticsearch instance
Caused by: org.elasticsearch.common.ParsingException: [search_response] failed to parse field [hits]
Caused by: com.fasterxml.jackson.core.JsonParseException: Unexpected character (':' (code 58)): was expecting comma to separate Array entries

Since each time reindexing process for this particular index reaches around 80% completeness and then the error is shown, I suspect there is a document in the index that's causing the problem.

How can I investigate this case? I was trying to find logged search responses (from source to target) but with no success. Source node is logging search requests, but these don't contain any valuable information.