Troubleshoot intermittent issue with PayPal REST API integration

We have a bespoke eCommerce system based on Wordpress CMS that integrates with PayPal via REST API. The purchase process worked fine on PayPal Sandbox, and fine for on Live for a few days. However, some users are now unable to complete a purchase due to a 401 error, seen in dev console. First:
"Error: Identitysecuretokenserv responded with status 401 for securityContext"

Then after clicking the "Pay Now" button in the PayPal popup:
"Error: Api: /smart/api/order/44N86558ED9246104 returned ack: permission_denied (Corr ID: f99146398ff0f)
{"ack":"permission_denied","message":"Insufficient privileges"}"

Not all users are affected, but those who are, consistently cannot purchase, regardless of browser or device used.

The token is generated at the last stage of checkout, so should be valid. Clearing the browser cache doesn't help.