I had this problem literally a few minutes ago. The problem is that you need to set async:false to async:true. I'm not sure exactly why this works, I guess because HTML5 is newer than XML.

Error is a bit different on this site but I think it's similar: JavaScript console.log causes error: "Synchronous XMLHttpRequest on the main thread is deprecated..."

Update

Hi, I'm back with some new and improved information. The Cross-Origin occurs with ports as well as domains/IPs, and will probably be in place with most decent browsers. If you want to know what is happening, try changing the IP to localhost or vice versa (if you are using localhost). Keep in mind this issue can occur when you are using different ports as well. For a quick fix, make sure that in the headers from whatever the back-end server is that you are putting out the correct Access-Control header response.addHeader("Access-Control-Allow-Origin", "*");.

Code is derived from this question

It was a Cross Origin Resource problem (CORS). My server was returning the correct information to me but my browser refused to accept it. To fix it I had to add 2 lines on my java server (not my site) to set them as the response header:

yourownvariable.setHeader("Access-Control-Allow-Origin:", "origin url of your site");yourownvariable.setHeader("Access-Control-Allow-Methods", "GET, POST,PUT");

I was having the same problem, with an identical error message. In my case it was caused by the server setting a Feature-Policy header with sync-xhr: none. Removing this directive fixed the problem.