Thanks Thomas for pushing me a step further. I might have solved my issues now, but will include my actions for the next person in need:
In Gardena.py, the config file was referred to with a relative path to the parent folder. That caused problems. Corrected that to a static path with great success.
The next error that occured was this message:
configparser.NoOptionError: No option 'api_key' in section: 'Gardena'
-Got that solved by adding that option to the cfg-file, with the same value as client_id.
I dont know if these two last errors is caused by a bug in the code or my behaviour, but "if it works it aint stupid"
Next error:
AttributeError: module 'websocket' has no attribute 'enableTrace'
I've installed the websocket package, but you'll need the websocket client: pip install websocket-client
That seems to have fixed the connection to the API.
The last error i experienced was domoticz not updating devices from the MQTT-stream. Easily solved:
make sure the topic in your config-file is set to
TOPIC=domoticz/in and not TOPIC=domoticz\in
