we had the identical issue. PayPal's customer support was completely unhelpful. The documentation shown here: https://developer.paypal.com/docs/integration/direct/vault/#pay-with-vaulted-card says that external_customer_id should be included if it was used when storing the card. The example given, includes it. After working fine for several years, stored card payment requests started failing in late January. Now, thanks to your posting, we have them working again.
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