pfSense python interpreter doen't read json

Question

I'm scratching my head. My goal is to get an input of json und put it out into an influxdb. Since' I'm gathering data from a pfSense my tools are limited. Therefor I'm using Python 3.8.

Developing on an Debian Machine first I wrote this code:

#!/usr/local/bin/python3.8
import os
import json
from influxdb import InfluxDBClient
from datetime import datetime

INTERFACES = ["igb0", "pppoe0"]
WORKING_FOLDER = "/tmp/json"
NOW = datetime.now()
CLIENT = InfluxDBClient("host", "8086", "user", "pw", "firewall")
for interface in INTERFACES:
    stats = {}
    os.popen(
        "vnstat --json d 2 -i {} > {}/{}_d.json".format(interface, WORKING_FOLDER, interface)
    ).read()
    os.popen(
        "vnstat --json m 2 -i {} > {}/{}_m.json".format(interface, WORKING_FOLDER, interface)
    ).read()

    with open(f"{WORKING_FOLDER}/{interface}_d.json", "r") as j:
        day = json.loads(j.read())
    with open(f"{WORKING_FOLDER}/{interface}_m.json", "r") as k:
        month = json.loads(k.read())

    if not len(day["interfaces"][0]["traffic"]["day"]) == 2:
        stats["yesterday"] = {"rx": 0, "tx": 0}
        stats["today"] = {
            "rx": day["interfaces"][0]["traffic"]["day"][0]["rx"],
            "tx": day["interfaces"][0]["traffic"]["day"][0]["tx"],
        }

    else:
        stats["yesterday"] = {
            "rx": day["interfaces"][0]["traffic"]["day"][0]["rx"],
            "tx": day["interfaces"][0]["traffic"]["day"][0]["tx"],
        }
        stats["today"] = {
            "rx": day["interfaces"][0]["traffic"]["day"][1]["rx"],
            "tx": day["interfaces"][0]["traffic"]["day"][1]["tx"],
        }

    if not len(month["interfaces"][0]["traffic"]["month"]) == 2:
        stats["last_month"] = {"rx": 0, "tx": 0}
        stats["this_month"] = {
            "rx": month["interfaces"][0]["traffic"]["month"][0]["rx"],
            "tx": month["interfaces"][0]["traffic"]["month"][0]["tx"],
        }

    else:
        stats["last_month"] = {
            "rx": month["interfaces"][0]["traffic"]["month"][0]["rx"],
            "tx": month["interfaces"][0]["traffic"]["month"][0]["tx"],
        }
        stats["this_month"] = {
            "rx": month["interfaces"][0]["traffic"]["month"][1]["rx"],
            "tx": month["interfaces"][0]["traffic"]["month"][1]["tx"],
        }

    json_body = [
        {
            "measurement": f"stats_{interface}",
            "time": NOW,
            "fields": {
                "yesterday_rx": stats["yesterday"]["rx"],
                "yesterday_tx": stats["yesterday"]["tx"],
                "yesterday_total": int(stats["yesterday"]["rx"]) + int(stats["yesterday"]["tx"]),
                "today_rx": stats["today"]["rx"],
                "today_tx": stats["today"]["tx"],
                "today_total": int(stats["today"]["rx"]) + int(stats["today"]["tx"]),
                "last_month_rx": stats["last_month"]["rx"],
                "last_month_tx": stats["last_month"]["tx"],
                "last_month_total": int(stats["last_month"]["rx"]) + int(stats["last_month"]["tx"]),
                "this_month_rx": stats["this_month"]["rx"],
                "this_month_tx": stats["this_month"]["tx"],
                "this_month_total": int(stats["this_month"]["rx"]) + int(stats["this_month"]["tx"]),
            },
        }
    ]

    CLIENT.write_points(json_body)

Testing it on my Debian machine (with line 12 & 13 commented out to not overwrite my json files) everything works like a charm. So I moved the script to my pfSense and I'm getting the following error:

[2.5.2-RELEASE][admin@pfSense]/usr/local/pythonscripts: ./trafficstats.py
Traceback (most recent call last):
  File "./trafficstats.py", line 18, in <module>
    month = json.loads(k.read())
  File "/usr/local/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/local/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/lib/python3.8/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

And I'm asking myself why. Because looking at the moth file is a valid json if I see correctly:

{"vnstatversion":"2.7","jsonversion":"2","interfaces":[{"name":"igb0","alias":"WAN","created":{"date":{"year":2022,"month":2,"day":4}},"updated":{"date":{"year":2022,"month":2,"day":12},"time":{"hour":9,"minute":30}},"traffic":{"total":{"rx":82416467756,"tx":43825701833},"month":[{"id":88,"date":{"year":2022,"month":2},"rx":82416467756,"tx":43825701833}]}}]}

Does anybody got a hint or idea whats going on?

Cheers, Gamie