Issue with API - 403 forbidden

I’ve setup phpList on a server hosted with 1&1. The server runs php version 7.4.23. I am also using the latest stable version of phpList (3.6.4). Everything seems to work fine using the UI (from subscribing to sending mail).

I’ve followed this guide to enable the API: API and integrations | phpList manual

I am able to login programmatically using the /api/v2/sessions endpoint (I receive a 201 response). I use the key returned from the response and generate an auth token in the following way: base64_encode([login_name]:[key]).

I authenticate by passing a basic authentication header (Authorization: Basic [token])

When I try to access any other endpoint (specifically, I need access to /api/v2/subscriptions to add a subscriber), I receive the following error:
{
“code”: 403,
“message”: “Forbidden”
}

I saw a few other posts of people getting the same error, but I have not seen a solution (ie. Problem with api: 403 forbidden - #5 by Thoma1s). Is this a configuration issue? If so, does anyone know how to resolve it?

Was able to resolve this issue. Solution is here: Problem with api: 403 forbidden - #11 by SirJohnPeters