Error downloading files writing to Response.Output

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Hello All.

I have seen many posts slightly similar to this question with no answer.
Maybe you have some FAQs or something, sorry if I looks repetitive.
Here is my Problem:

In my application I have a link button inside a module on an IBuySpy Portal
to download an excel file. I generate the excel file using the Office 2003
XML file format. There is to say that my excel file is an XML document. The
link button code looks like this:

Page.Response.C lear();
Page.Response.C learContent();
Page.Response.C ontentType = "applicatio n/vnd.ms-excel";
Page.Response.A ppendHeader("Co ntent-Disposition",
"attachment;fil ename=ExcelFile .xls");
Page.Response.C ontentEncoding = System.Text.Enc oding.UTF8;
Page.Response.O utput.Write(exc elXML);
Page.Response.E nd();

I installed the module in an IBuySpy Portal and this was working fine: IE
opens a download file dialog prompting to open or save it and both options
was working well.

Suddenly, it stops work; if I click on “Open� I got some error: “C:\Document
and Settings\...\Lo calSettings\Tem porary Internet Files\Cont….. \
ExcelFile.xls†� could not be found. Check the spelling of the file name, and
verify that the file name is correct.� If I click on save and then open it by
hand it works.

This also happens on other XML files (not excel files, just personal XML
files).

Any idea on this? this is killing me :(

Thank you.