401 Unauthorized: [no body]

We have a Java-based application that interfaces with Acrobat Sign.

We have an Access Token (expired) and a Refresh Token (not expired). 

When we attempt to use the Refresh Token to get a new Access Token in Production, we get a 401 Unauthorized: [no body] error. 

Here's what we've tried:

1. Taking the exact Client ID, Client Secret, Access Token, and Refresh Token from Production database tables and manually doing the POST via cUrl - this works, we get a new Access Token
2. Repeating #1 but with Postman - this also works
3. Copying the database rows down to a Dev environment and running the same code as Production - this also works

Is it possible that Acrobat Sign blocked our Prod Server?  I know for OAuth Code Flow, we have to whitelist the Redirect URI, but I thought that calls to get a new access token from endpoint: https://api.eu1.documents.adobe.com/oauth/v2/refresh

could be done from anywhere without whitelist.  

Any thoughts/suggestions?