Program runs with sudo and root, not with systemd

Question

I have an application that ran for months on Stretch and Python 2.7 and 3.5, on a Raspberry Pi Model 3B+, however when I moved it to Buster and Python 3.7 I ran into a very strange problem. The error message I get is "error saving session_id: write() argument 1 must be unicode, not str" from a json.dump save of a list.

(code: json.dump(client.user_data, fout))

If I run the script as sudo, to test for the eventual running as root by systemd, it works fine. When I run it as root through sudo su, it also works fine. When I run it through systemd I get the error. For many hours I have been going in circles trying to zoom in on the problem, but now I'm baffled.

At first I though that the issue was with the unicode changes in Python, so I tried to eliminate that as a source of the problem. I even went as far as to add a function to really make sure I'm dealing with unicode, and not strings. (function utfy_dict)

The list I'm using and trying to save looks like this, with some fields edited for pricacy: (taken from a print(data) output":

{u'sessionId': u'555C703C-607x-bla-bla', u'userInfo': {u'username': u'my_email@gmail.com', u'city': u'my_city', u'isActivated': True, u'firstname': u'Paul', u'lastname': u'mylastname', u'userID': my userId, u'zipcode': u'my Zipcode', u'telephone': u'', u'deviceCount': 0, u'streetAddress': u'my street', u'securityQuestion3': u'NotUsed', u'securityQuestion2': u'NotUsed', u'securityQuestion1': u'NotUsed', u'country': u'NL', u'userLanguage': u'nl-NL', u'tenantID': 5, u'latestEulaAccepted': False}}

The relevant pieces:

        try:
            # save the session-id so we don't need to re-authenticate every polling cycle
            print("save session_id")
            # here comes the offending piece: an exception is raised
            utfy_dict(client.user_data)
            print(client.user_data)
            print("\n")
            with io.open(session_id, "w", encoding="utf-8") as fout:
                json.dump(client.user_data, fout)
        except Exception as error:# (IOError, ValueError,TypeError):
            write_log("error", "error saving session_id: {}".format(error))

    return

def utfy_dict(dic):
    '''
    https://stackoverflow.com/questions/33699343/convert-every-dictionary-value-to-utf-8-dictionary-comprehension
    '''
    if isinstance(dic,bytes):
        return(dic.encode("utf-8"))  # the key is this!
    elif isinstance(dic,dict):
        for key in dic:
            dic[key] = utfy_dict(dic[key])
        return(dic)
    elif isinstance(dic,list):
        new_l = []
        for e in dic:
            new_l.append(utfy_dict(e))
        return(new_l)
    else:
        return(dic)

Systemd service file:

# This service installs a Python script that monitors the CV temperatures.

[Unit]
Description=Installing the cv_mon monitoring script
Requires=basic.target
After=multi-user.target

[Service]
ExecStart=/usr/bin/python /home/pi/cv_app.py
Restart=always
# wait for 10s before the SIGKILL gets send
TimeoutStopSec=10s

# The number of times the service is restarted within a time period can be set
# If that condition is met, the RPi is rebooted
#
StartLimitBurst=4
StartLimitInterval=180s
# actions can be none|reboot|reboot-force|reboot-immidiate
StartLimitAction=none

[Install]
WantedBy=multi-user.target

The get_evo_data function gets called every three minutes. I would like to reiterate that the code ran fine for more than a year on another platform, and it also runs fine as sudo or root, just not under systemd.

Here is an error report: (note the reference to Python 2.7/json ???)

2019-07-31 00:15:30,095 ERROR    JSON decode error reading evo_saved_data
2019-07-31 00:15:30,099 ERROR    Got exception: write() argument 1 must be unicode, not str
2019-07-31 00:15:30,102 INFO     Traceback (most recent call last):
  File "/home/pi/cv_app.py", line 717, in main
    get_evo_data()
  File "/home/pi/cv_app.py", line 314, in get_evo_data
    json.dump(client.user_data, fout)
  File "/usr/lib/python2.7/json/__init__.py", line 190, in dump
    fp.write(chunk)
TypeError: write() argument 1 must be unicode, not str