When calling "dbutils.fs.mount" the proxy call towards the cluster is using the wrong dictionary names for the parameters.
The generated code looks like this:

        import json
        (args, kwargs) = json.loads('[[], {"source": "<someUrl>", "mountPoint": "/mnt/<myMountPoint", "encryptionType": "", "owner": "", "extraConfigs": null}]')
        result = dbutils.fs.mount(*args, **kwargs)
        dbutils.notebook.exit(json.dumps(result))

When this is executed in the cluster, the following error is thrown:
TypeError: DBUtils.FSHandler.mount() got an unexpected keyword argument 'mountPoint'
Further more this error is not proxied back - instead you get TypeError: the JSON object must be str, bytes or bytearray, not NoneType (dbutils.py:245)

The actual problem are the wrong parameter names in the Method-Signatur of "mount".
The original DataBricks-API expects the parameters with underscores instead of camelCase (see https://docs.databricks.com/dbfs/mounts.html):

dbutils.fs.mount(
  source: str,
  mount_point: str,
  encryption_type: Optional[str] = "",
  extra_configs: Optional[dict[str:str]] = None
)

Same probably applies to the other Mount-Methods.