3D FlipBook - Flipbooks have stopped working with message UnknownErrorException: Failed to fetch

I've been using Flipbooks successfully on a couple of sites for months. But they have stopped working on one site with error message: UnknownErrorException: Failed to fetch

The console says Access to fetch at '[the PDF URL]' from origin '[my website]' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

I have tried turning off all plugins and changing the theme but still getting the message.