Thank you for your update. I'm glad to hear that you have found a workaround by creating a new project and generating new credentials. I'm happy to investigate further why your old credentials don't work on the Merchandise Refund API (https://sandbox.api.visa.com/visadirect/mvisa/v1/mr). Can you please send an email to firstname.lastname@example.org and provide the following information.
1. attach certificate public key PEM file
2. attach certificate private key PEM file
Can you share the solution that worked for you? We are facing the same problem, but in the production environment. Some cards got the answer, for others we just got this error.
Please send an email to email@example.com and share the following:
1. Project name in production environment
2. Country/region the project was launched in
3. End Point URL
4. Request Header
5. Request Body
6. Response Header (include the x-correlation-id)
7. Response Body