API User: Access denied due to invalid subscription key Error

I have an active Collections subscription in the MTN MoMo Developer sandbox. Both the Primary and Secondary Collections subscription keys return HTTP 401 when calling POST /v1_0/apiuser on the sandbox API. The response is: “Access denied due to invalid subscription key. Make sure to provide a valid key for an active subscription.” DNS and HTTPS connectivity to the sandbox endpoint are working. Please verify or reactivate the Collections sandbox subscription/API Management keys for my developer account.