Notebooks on EMR (AWS): Failed to start kernel

I am creating a notebook with the built-in option from "Amazon EMR" following the steps in the official documentation

• Everything as default
• EMR-Cluster created with the notebook-Wizzard

After starting successfully the notebook in Jupyter, an error appears by loading the kernel. Different kernels (Python3, PySpark) produce the same result.By clicking in "Kernel Error" on the toolbar in Jupyter, following message is shown:

Failed to start kernel

"Error attempting to connect to Gateway server url 'http://localhost:17777'. Ensure gateway url is valid and the Gateway instance is running."

Any clue what am I missing?

Thanks!