Encoding problem with custom MQTT broker app

I have installed a smart home installation for which there is no integration. Using its OpenAPI, I have written a small program that transmits weather data from the weather station using MQTT sensors, for example. This works wonderfully except for the display.

For “unit_of_measurement” I transmit the string “°C” in the “config” message. This works perfectly on my dev workstation (Windows 11). Ultimately, however, the program runs on Linux in an lxc container (finally as a Docker container).

Windows11 started: 21.8 °C
Linux: 21.8 ??C

What is the problem? How do I solve it?

edit:
In addition I have many warnings in the home assistant log like these:

2024-06-10 21:57:14.286 WARNING (MainThread) [homeassistant.components.mqtt.sensor] Invalid undecoded state message ‘b’41.6’’ received from ‘homeassistant/sensor/WeatherStation5075_MaxTemp’

2024-06-10 21:57:14.387 WARNING (MainThread) [homeassistant.components.mqtt.sensor] Invalid undecoded state message ‘b’25.0’’ received from ‘homeassistant/sensor/WeatherStation5075_MaxTempToday’

2024-06-10 21:57:14.488 WARNING (MainThread) [homeassistant.components.mqtt.sensor] Invalid undecoded state message ‘b’-8.9’’ received from ‘homeassistant/sensor/WeatherStation5075_MinTemp’

2024-06-10 21:57:14.589 WARNING (MainThread) [homeassistant.components.mqtt.sensor] Invalid undecoded state message ‘b’17.0’’ received from ‘homeassistant/sensor/WeatherStation5075_MinTempToday’

2024-06-10 21:57:14.690 WARNING (MainThread) [homeassistant.components.mqtt.sensor] Invalid undecoded state message ‘b’126.0’’ received from ‘homeassistant/sensor/WeatherStation5075_MaxWind’

2024-06-10 21:57:14.790 WARNING (MainThread) [homeassistant.components.mqtt.sensor] Invalid undecoded state message ‘b’9.4’’ received from ‘homeassistant/sensor/WeatherStation5075_MaxWindToday’