I had faced the same issue, somewhere deep in our code we had this pseudocode:
onload of iframe submit a form
After 2 seconds, remove the iframe
thus, when the server takes more than 2 seconds to respond the iframe to which the server was writing the response to, was removed, but the response was still to be written , but there was no iframe to write , thus chrome cancelled the request, thus to avoid this I made sure that the iframe is removed only after the response is over, or you can change the target to "_blank". Thus one of the reason is: when the resource(iframe in my case) that you are writing something in, is removed or deleted before you stop writing to it, the request will be cancelled