Reindex parsing failure how to debug

I tried to reindex today with the following:

POST _reindex
{
  "source": {
    "index": "var_log_nginx_access-2020.03.16"
  },
  "dest": {
    "index": "var_log_nginx_access-2020.03.16-new"
  }
}

After this I get the following message:

 {
      "index" : "var_log_nginx_access-2020.03.16-new",
      "type" : "_doc",
      "id" : "lv2x43AB4pUu6mpyqbN0",
      "cause" : {
        "type" : "mapper_parsing_exception",
        "reason" : "failed to parse field [timestamp] of type [date] in document with id 'lv2x43AB4pUu6mpyqbN0'. Preview of field's value: '16/Mar/2020:15:13:45 +0100'",
        "caused_by" : {
          "type" : "illegal_argument_exception",
          "reason" : "failed to parse date field [16/Mar/2020:15:13:45 +0100] with format [strict_date_optional_time||epoch_millis]",
          "caused_by" : {
            "type" : "date_time_parse_exception",
            "reason" : "Failed to parse with all enclosed parsers"
          }
        }
      },
      "status" : 400
    },

This is what the mapping of the old index looks like:

{
  "mapping": {
    "_doc": {
      "properties": {
        "@timestamp": {
          "type": "date"
        },
        "@version": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "ClientIP": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "Error_Message": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "agent": {
          "properties": {
            "ephemeral_id": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "hostname": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "id": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "name": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "type": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "version": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }
          }
        },
        "connection_id": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "ecs": {
          "properties": {
            "version": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }
          }
        },
        "host": {
          "properties": {
            "name": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }
          }
        },
        "input": {
          "properties": {
            "type": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            }
          }
        },
        "log": {
          "properties": {
            "file": {
              "properties": {
                "path": {
                  "type": "text",
                  "fields": {
                    "keyword": {
                      "type": "keyword",
                      "ignore_above": 256
                    }
                  }
                }
              }
            },
            "flags": {
              "type": "text",
              "fields": {
                "keyword": {
                  "type": "keyword",
                  "ignore_above": 256
                }
              }
            },
            "offset": {
              "type": "long"
            }
          }
        },
        "loglevel": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "message": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "method": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "process_id": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "request": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "tags": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "thread_id": {
          "type": "text",
          "fields": {
            "keyword": {
              "type": "keyword",
              "ignore_above": 256
            }
          }
        },
        "timestamp_nginx": {
          "type": "date",
          "format": "yyyy/MM/dd HH:mm:ss||yyyy/MM/dd||epoch_millis"
        }
      }
    }
  }
}

This is the mapping of the newly created index:

{
  "_doc": {
    "_meta": {},
    "_source": {},
    "properties": {
      "request": {
        "type": "text"
      },
      "ClientIP_access": {
        "type": "ip"
      },
      "http_referer": {
        "type": "text"
      },
      "bytes_sent": {
        "coerce": true,
        "index": true,
        "ignore_malformed": false,
        "store": false,
        "type": "byte",
        "doc_values": true
      },
      "timestamp": {
        "type": "date"
      },
      "status": {
        "coerce": true,
        "index": true,
        "ignore_malformed": false,
        "store": false,
        "type": "integer",
        "doc_values": true
      },
      "http_user_agent": {
        "type": "text"
      }
    }
  }
}

This is the grok filter pattern:

NGINX_ACCESS_DATE %{MONTHDAY}/%{MONTH}/%{YEAR}:%{TIME} %{ISO8601_TIMEZONE}
NGINX_ACCESS %{IP:ClientIP_access} - - \[%{NGINX_ACCESS_DATE:timestamp}\] %{QS:request} %{INT:status} %{INT:bytes_sent} %{QS:http_referer} %{QS:http_user_agent}

What can I do to debug this or is there something I have done wrong.

My motivation for this is because I wanted to create a visualisation using ipv4 range but I noticed that my field "ClientIP_access was not mapped to the "IP" type. With the new index that I checked this morning the mapping shows that the IP type and the field are mapped but actually it still complains there is not a valid field available for this.

As much help would be great, many thanks