Can't start Plaid quickstart app in development mode – Nginx

I’ve just started using Plaid. I’ve set up the quickstart app locally and it runs fine in sandbox mode. The Link panel loads fine, and I can log into an institution using the sandbox creds.

My .env file looks like this:

PLAID_CLIENT_ID=xxxxxxxx
PLAID_SECRET=xxxxxxxx
PLAID_ENV=sandbox
PLAID_PRODUCTS=transactions
PLAID_COUNTRY_CODES=US,CA
PLAID_REDIRECT_URI=http://localhost:3000

I’d now like to check that Plaid will work with a few particular real/live institutions, so I’m attempting to update my quickstart app to work in development mode. To do this I am simply updating PLAID_SECRET to be my development secret key, and PLAID_ENV to be ‘development’.

When I now start the app, the start page no longer shows the ‘Launch Link’ button, but instead shows:

The logs suggest the issue is happening on the POST /api/create_link_token call. If I remove the error handling from that route function the logs show me the issue:

Traceback (most recent call last):
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "server.py", line 196, in create_link_token
    response = client.link_token_create(request)
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api_client.py", line 769, in __call__
    return self.callable(self, *args, **kwargs)
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api/plaid_api.py", line 5566, in __link_token_create
    return self.call_with_http_info(**kwargs)
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api_client.py", line 831, in call_with_http_info
    return self.api_client.call_api(
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api_client.py", line 406, in call_api
    return self.__call_api(resource_path, method,
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api_client.py", line 200, in __call_api
    raise e
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api_client.py", line 193, in __call_api
    response_data = self.request(
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/api_client.py", line 452, in request
    return self.rest_client.POST(url,
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/rest.py", line 264, in POST
    return self.request("POST", url,
  File "/Users/william/Personal/quickstart/python/venv/lib/python3.8/site-packages/plaid/rest.py", line 223, in request
    raise ApiException(http_resp=r)
plaid.exceptions.ApiException: (400)
Reason: Bad Request
HTTP response headers: HTTPHeaderDict({'Server': 'nginx', 'Date': 'Sun, 16 May 2021 18:22:05 GMT', 'Content-Type': 'application/json; charset=utf-8', 'Content-Length': '301', 'Connection': 'keep-alive'})
HTTP response body: {
  "display_message": null,
  "documentation_url": "https://plaid.com/docs/?ref=error#invalid-input-errors",
  "error_code": "INVALID_API_KEYS",
  "error_message": "invalid client_id or secret provided",
  "error_type": "INVALID_INPUT",
  "request_id": "EbCTgZkFdeYQQ8r",
  "suggested_action": null
}

So it’s suggesting I’m using the wrong client id or secret, but I don’t understand. The client ID should be fine (it worked in sandbox mode), and the secret is definitely my secret for development mode (I’ve even tried generating a new one but no difference). Am I missing something?

Finally, if I use Postman to POST to https://development.plaid.com/link/token/create to create a link token, using the same client ID and ‘development’ secret, it seems to work fine (i.e. I get a 200 response and brand new link token).

Any help is much appreciated!