This error will be logged when Elasticsearch tries to index data that is an object, into a field that is not mapped as one. Or in reverse, if you try to index something that isn't an object into a field that is mapped as one.
Common sources of this error are from data being sent from Filebeat or Logstash. This thread has two good examples - Logstash Errors mapper_parsing_exception vs illegal_argument_exception
You will need to check the mapping for the related index, and compare that to the value that is being sent to Elasticsearch for indexing. If you are using Filebeat/Logstash, you may need to adjust any processing of the events that they are doing, and this post has a good example of what you can do.