Locally works but on the Nginx server I get: `Unable to render this definition. The provided definition does not specify a valid version field`

Question

I have a Flask / Flask-Smorest application which when deployed locally the swagger UI and Open API show correctly. However, when deployed on the Cloud onto Nginx then it produces the output:

Unable to render this definition

The provided definition does not specify a valid version field.

Please indicate a valid Swagger or OpenAPI version field. Supported version fields are swagger: "2.0" and those that match openapi: "3.x.y" (for example, openapi: 3.1.0)

I have tried with two different approaches.

==================== FIRST APPROACH ====================

The first approach is a copy paste of the Flask-Smorest documentation OpenAPI Flask-Smorest:

 class MyConfig:
     OPENAPI_VERSION = '3.0.3'
     OPENAPI_URL_PREFIX = '/docs'
     OPENAPI_SWAGGER_UI_PATH = ''
     # I tried with each of the possibilities given   
     OPENAPI_SWAGGER_UI_URL = 'https://cdn.jsdelivr.net/npm/swagger-ui-dist/' 

I even ensure that the endpoint is covered by CORS to avoid any issues there:

 from flask import Flask
 from flask_cors import CORS
 from flask_smorest import Api

 app = Flask(__name__)
 CORS(app, resources={'/api/*': {'origins': '*'}, '/api/*': {'docs': '*'}})
 api = Api(app)
 cache.init_app(app)

==================== SECOND APPROACH ====================

I downloaded and extracted Swagger UI latest v5.9.0 under the source folder static\swagger-ui. Then changed the application from above in the following ways:

 class MyConfig:
     OPENAPI_VERSION = '3.0.3'
     OPENAPI_URL_PREFIX = '/docs'
     OPENAPI_SWAGGER_UI_PATH = ''
     OPENAPI_SWAGGER_UI_URL = '/static/swagger-ui' 
 
 @app.route('static/<path:path>') 
 def serve_static(path):
     return send_from_directory('static', path)

Both approaches work perfectly locally, the later approach showing a much fancier Swagger interface. But both approaches fail on the Nginx server with the same error. I then inspected the swagger json file referenced by swagger-initializer.js: https://petstore.swagger.io/v2/swagger.json

which indeed does include the version in the first line: "swagger":"2.0" so I don't understand why I'm getting this error.