I've hit the same problem and isolated it to some degree. I can reproduce it in our main repo (with the python functions & UDF wrappers installed as part of a package), but cannot reproduce it in a new minimal repo I made for testing. When I copy the package source in to a non-repo folder, everything works fine. 

Same type of error messages of:

...
ValueError: cannot configure default credentials, please check https://docs.databricks.com/en/dev-tools/auth.html#databricks-client-unified-authentication to configure credentials for your preferred authentication method.
...
ModuleNotFoundError: No module named 'dbruntime'
...

 I don't understand the RCA of that github issue or why setting the environment variable may help.

I'm working with databricks support to resolve, and will try to share answers here.