What went wrong?

The CORS request requires preflight, preflighting could not be performed. There are a couple of reasons why preflighting might fail:

• A cross-site request has previously been performed that already did a preflight, and doing the preflight again is not permitted. Make sure your code only preflights once per connection.
• The preflight request suffered any kind of networking error that might ordinarily occur.

See also

• CORS errors
• Glossary: CORS
• CORS introduction