FineUploader Exception : XhrReadyState=0 and XhrResponseStatus=0 - amazon-s3

Question

We're experiencing FineUploader exceptions with XhrReadyState=0 and XhrResponseStatus=0 on 5.4.1 in our production environment intermittently across browsers. We've checked out our CORS configuration for our S3 bucket and we do have the wild card origin header set.
Access-Control-Allow-Origin: *
Was looking at FineUploader CORS selectively working, and we do not set the sendCredentials value in our configuration.
Error from fine uploader is
XHR returned response code 0
The XHR response object contains the following values,
XhrReadyState=0
XhrResponseText=empty
XhrResponseType=empty
XhrResponseStatus=0
XhrTimeout=0
XhrAllResponseHeaders=empty
Update 1
So we captured a repro of this. It looks like one of our OPTIONS requests is getting a 403 randomly in the middle of the upload.
[Error] Failed to load resource: the server responded with a status of 403 (Forbidden) (a7db116d-26ec-4b45-9566-8697d42a7b4c.VOB, line 0)
[Error] Failed to load resource: Origin http://www.ourdomain.com is not allowed by Access-Control-Allow-Origin. (a7db116d-26ec-4b45-9566-8697d42a7b4c.VOB, line 0)
[Error] XMLHttpRequest cannot load http://ourbucket.s3.amazonaws.com/....
Origin http://www.ourdomain.com is not allowed by Access-Control-Allow-Origin.
Here is the screen grab from the Safari console. I was suspecting a network problem, but I wouldn't expect such a quick 403 response if a network issue was . Also, previous OPTIONS requests had just succeeded before and after the one that 403'd. We have configured CORS on our S3 bucket.

Answer 1

Status 0 indicates some issue either with your server or with the network. More specifically, it indicates that the response received was completely empty. This can be caused by CORS issues or even a temporary network issue. You'll want to examine these possibilities further in order to determine what is causing the issue for you.