Agile Central - App SDK: What does this error mean: "XMLHttpRequest cannot load XXX No 'Access-Control-Allow-Origin' header"?

Document ID : KB000126462
Last Modified Date : 11/02/2019
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Question:
When developing an SDK app we're getting this error: "XMLHttpRequest cannot load XXX No 'Access-Control-Allow-Origin' header".
We seem to continue to get this error despite stopping our debugger and even rebooting the machine. 

What does this error mean? How can we fix it?
Answer:

The Access-Control-Allow-Origin header referred to in the error message is part of the CORS standard which allows our server to explicitly grant permission to your SDK development site, so that your browser can access the server's data.


A basic implementation would just include: Access-Control-Allow-Origin: *     in order to permit any website to read the data. For example:

Access-Control-Allow-Origin: http://example.com/

Our Rally site already implements this CORS permission. 


While we don't know the root cause of this error, here is what you can do to fix it:
1. Clear out your browser cache, close your browser, then reopen it and rerun your code. Most times this is enough and it will fix this error.
2. If the first step didn't fix this issue then open the developer tools in Chrome and check the "disable cache when open" flag.