This happens when you access the download link from a different browser than the one in which you are logged in to you CaptainForm account. For example, let’s say that you normally use browser A when working with your forms. But browser B is set as default for opening email links. The download link will not work, because it will open up in browser B, whilst you are authenticated on CaptainForm account in browser A.
Related documentation: Fixing CSV Export in Microsoft Excel