If you've a logstash filter that contains a json filter/decoding step like

filter { json { source => "log" } }

this, and you end up with an error message like that:

[2019-06-21T09:47:58,243][WARN ][logstash.filters.json ] Error parsing json {:source=>"log", :raw=>{"file"=>{"path"=>"/var/lib/docker/containers/abdf3db21fca8e1dc17c888d4aa661fe16ae4371355215157cf7c4fc91b8ea4b/abdf3db21fca8e1dc17c888d4aa661fe16ae4371355215157cf7c4fc91b8ea4b-json.log"}}, :exception=>java.lang.ClassCastException}

It might be just telling you that the field log actually does contain valid json, and no decoding is required.