Hi Pbanerji,
Is your site online? If so, can you post the uploader page url here? So we can check it for you.
Please try use IFrame upload mode, it should shows you move detail about the error.
<CuteWebUI:UploadAttachments ID="uploader1" runat="server" UploadType="IFrame">
</CuteWebUI:UploadAttachments>
Does this error happens with all files? Or just large file?
Regards,
Ken