Using Google Cloud Endpoint, why is there a static/proxy.html used for certain requests?

We're using a Google Cloud Java Endpoint with a Javascript Client and we've noticed that in some cases, requests, originating from IE11, seem to be go thru the cloud endpoint static proxy (http://ift.tt/29UMPcC) -- as the HTTP referrer (referer).

Why?

Furthermore, when those requests fail, they do with the follow error:

response: { status: 0 statusText: No status text available body: {"error":{"code":-1,"message":"A network error occurred, and the request could not be completed."}} }

Any help would be appreciated!

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