From the apache ingest pipeline (link):

  - rename:
      field: message
      target_field: event.original

The rename processor documentation states:

If the field doesn’t exist or the new name is already used, an exception will be thrown.

This means that any document that already has an "event.original" field (with or without a "message") field will cause an ingestion error:

"message": "......",
"error": {
"message": "field [event.original] already exists"
},

A suggestion is to tolerate the presence of "event.original" and "message" fields by including an if condition in the rename processor.