Which IE version are you using? Is the upload target a HTTPS URL?
Do you *always* see "SEC7111: HTTPS security is compromised by res://ieframe.dll/dnserrordiagoff.htm" in the console, or only in the failing case? That error message means that the HTTPS page had a frame which attempted to navigate (e.g. to perform an upload) to a server which was unavailable (either due to a DNS lookup failure or potentially due to a HTTPS connection problem or timeout).
Does your server have an upload timeout? In very old versions of IE (not the one you're using), the upload send buffer was too small so files uploaded very slowly. Running Fiddler fixed that problem because Fiddler used a better send buffer size than IE6/7 (see http://blogs.telerik.com/fiddler/posts/13-02-28/help!-running-fiddler-fixes-my-app-
for more details on this.).