I've been trying to get email before download (version 2.5.1) to work with a single download. After filling out the form and clicking the download link, I'm only presented an empty page.
After quite some investigation, I found out that the ebd_link table is never filled in, so my download can never be found and cannot be downloaded.
Some more investigation showed that in email-before-download.php on lines 366-371, you forget to fill in field selected_id. This may not be used at all, but your current table description says this is a required field (i.e. it cannot be NULL but has no default value). I've added this manually, setting it to 0 (don't know if that's the correct value), and now the download is working as advertised.