Flask-GoogleMaps returns 403 Forbidden error

I'm a newbie writing a Flask web app that displays in a Google Map a series of locations stored in a SQLite database. In order to do this I'm trying to use the Flask-GoogleMaps extension (https://github.com/flask-extensions/Flask-GoogleMaps), but even when I only try the example code given in the documentation the map won't display in the page. The server returns a 403 error:

GET http://maps.googleapis.com/maps/api/mapsjs/gen_204?csp_test=true 403 (Forbidden).

I know the Google Maps JS API key given is correct since the map displayed just fine when I included the raw code as explained in the Google Maps API documentation.

This is the app that I'm running:

from flask import Flask, render_template
from flask_googlemaps import GoogleMaps, Map

app = Flask(__name__)

GoogleMaps(app, key="MY_KEY")

@app.route("/")
def mapview():
    # creating a map in the view
    mymap = Map(
        identifier="view-side",
        lat=37.4419,
        lng=-122.1419,
        markers=[(37.4419, -122.1419)]
    )
    return render_template('example.html', mymap=mymap)

if __name__ == "__main__":
    app.run(debug=True)

This is the rendered HTML:

<!DOCTYPE html>
<html>
  <head>
    {{mymap.js}}
  </head>
  <body>
    <h1>Flask Google Maps Example</h1>
    {{mymap.html}}
  </body>
</html>

What could be the problem? Thank you very much in advance!