IE is much more sensitive than other browsers about changing the DOM after sending headers, but before the page ends.
Here is your problem:
Response.Redirect("../Default.aspx?Sid=1", false);
Try changing false to true.
Also, be very careful with the cover in the names of your pages. "Default.aspx" and "default.aspx" are not really the same page, even if Windows allows you to handle it.
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