What causes the MUTUAL AUTHENTICAtION FAILED error?

Payment intent calls to the Payment API may start to fail for no apparent reason. There are no error logs for FinDock or Salesforce, but admins and developers of the external website connection see the error MUTUAL_AUTHENTICATION_FAILED.

This error likely indicates the password of the Salesforce integration user for the Payment API integration needs to be reset. This solution has been confirmed by the Trailblazer Community.