Good to hear that it's solved. The API key is linked to a user account and the snapshot endpoint will only work for accounts who have access to that camera. If the camera is moved to a network that the account has no access to, this could be the reason for it to stop working. However just regenerating a new API key shouldn't make a difference then. Maybe there was a different issue.
Since the snapshot API uses POST another possible explanation could be the well-known redirect issue. Postman handles these correctly, but not all libraries do. So in case you run into the issue again, try replacing the url by the direct shard url:
Note that the request is redirected to n##.meraki.com. A 302 may happen on any API call, including those that modify state such as DELETE, POST, and PUT. When a GET is redirected, the status code will be 302. When a non-GET is redirected, the status code will be 307 for Postman, or 308 for any other client. Client applications must follow these redirects. No action will be taken by the host issuing the redirect, so even non-idempotent requests are safe to retry on the new host.