How to resolve '403' errors during mTLS handshake in a payments app?

[sahil_197]

We are a payments aggregator currently working on creating a payments app (link - https://shopify.dev/apps/payments/creating-a-payments-app)

We have connected our app to a Shopify developer store and are currently logging '403' errors during the mTLS handshake with the error message 'ERR_NO_CLIENT_CERT'. This seems to be indicating that either Shopify is not passing their client certificate OR the certificate keys may have rotated. 

We have configured the Shopify certificate chain listed on the link - https://shopify.dev/apps/payments#mtls-configuration. Moreover, our app was working with the developer store ~3months ago with the same mTLS configuration. 

Has anyone else faced/facing similar issues? Please suggest on the possible steps.