Reason: CORS request did not succeed

What went wrong?

The HTTP request which makes use of CORS failed because the HTTP connection failed at either the network or protocol level. The error is not directly related to CORS, but is a fundamental network error of some kind.

In many cases, it is caused by a browser plugin (e.g., an ad blocker or privacy protector) blocking the request.

How to fix

• Check DevTools > Network to see whether the request failed due to DNS resolution, a timeout, a connection being refused, or a TLS handshake error.
• Disable browser extensions or try a private browsing window, as ad blockers, firewalls, and privacy tools can block network requests.
• Fix certificate or TLS issues, such as expired or invalid certificates.
• Avoid mixed content: if the page is loaded over HTTPS, requests to HTTP resources can fail. Serve the API over HTTPS instead.
• Confirm that the server is responding correctly and that the endpoint returns a response.

If you are using a local dev server, ensure that the correct scheme and port are used and that the service is running.

Other possible causes include:

• Trying to access an https resource that has an invalid certificate will cause this error.
• Trying to access an http resource from a page with an https origin will also cause this error.
• From Firefox 68 to Firefox 84, https pages were not permitted to access http://localhost. This has been changed with Bug 1488740.
• The server did not respond to the actual request (even if it responded to the Preflight request). One scenario might be an HTTP service being developed that panicked without returning any data.
• The window is in "Private Browsing" mode (which may have security requirements that could block a CORS request).

See also

• CORS errors
• Glossary: CORS
• CORS introduction