Hello Jeffrey,
Unless you are exporting more than 16000 accounts, it's unlikely any data has been lost. The export control formats the data according to a very old version of Excel, and the newer versions seem to generate this message every time they open the file.
Regarding the PS - when you open the CSV file in Excel, tell it that the column containing your account numbers contains text. Then the figures will come in properly. This is a common problem when loading CSV files into Excel. It's more commonly noticed with telephone numbers.
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Andrew Bates