Error: Content-Type is not allowed by Access-Control-Allow-Headers

I solved the problem adding to Apache Web Server virtual host configuration the following settings

Header set Access-Control-Allow-Origin "*"
Header set Access-Control-Allow-Headers "Origin, X-Requested-With, Content-Type, Accept"

Solution for PHP:

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: POST,GET,OPTIONS');
header('Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept');

(Need to send that before any other content)

Set up CORS (Cross-site HTTP Requests) in node. For me it looks like the following:

app.use('/api', function(req, res, next) {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type');
  next();
});

I had the same problem and I solved it by adding the following header: Access-Control-Allow-Headers: content-type