Getting "Can't read from file" error on online.swagger.io

{
    "schemaValidationMessages": [{
        "level": "error",
        "message": "Can't read from file https://api.npr.org/documentation/beryllium/swagger.json"
    }]
}

https://api.npr.org/documentation/beryllium/swagger.json is accessible to the public, and I know that the validator used to work at least as of March 1st, when I last did some maintenance on our Swagger instance. (I opened a ticket in our system to follow up with some more maintenance for a future date, which is how I know when it was last confirmed working.) So sometime between then & today, the validator stopped working.

Any ideas?

Start by reproducing the reported error for https://api.npr.org/documentation/beryllium/swagger.json through online.swagger.io and compare it with the URL's public accessibility. Trace where the validator reads remote files; done means identifying the cause of the rejection and restoring successful validation for this document.