Hi @andreu,
Thanks for sharing that error. Could you please share a copy of your site’s System Status? You can find it via WooCommerce > Status. Select “Get system report” and then “Copy for support”.? Once you’ve done that, paste it here in your response.
You’ve said your API keys appear correct, so beyond that would you happen to be switching between test and live mode at the moment? I ask as this could come up when saving a card while Stripe is in test mode and then trying to switch to live and use that same saved card.
Ultimately the error is saying a source doesn’t exist in the current Stripe account (API Keys) the site is using. So it could be worth triple checking the API Keys one more time, and then thinking if potentially a card that was saved or you are attempting to use may have happened under test mode or even another Stripe account.
Would any of those be a possibility?