What is causing the pre-flight request CORS error in IIS?

edit

The pre-flight request CORS error in IIS is caused by the Cross-Origin Resource Sharing (CORS) policy implemented in modern browsers. This policy restricts web pages from making requests to a different domain than the one that served the original page. When a web page attempts to make a cross-domain request, the browser first sends a pre-flight request to the server to determine if the request is allowed according to the CORS rules. If the server does not respond correctly to this pre-flight request, the browser will block the request and generate a CORS error.