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 email@example.com and provide the following information.
1. attach certificate public key PEM file
2. attach certificate private key PEM file
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