I would have loved this --- bearing in mind XenForo's inadequacies in this area --- but I got one of those cutsie Oops warnings, which said: 'All banned_email values must be unique.'
I daresay there were addresses already listed in the filter on this old, but new to XenForo, forum; but tolerances should be loose enough to *allow* duplicates instead of stalling the install. No-one is going to manually cross-check 3000 odd addresses with 3000 odd other addresses.
The author is excellent. It is XenForo screwing up.
On my installation, any existing banned email records remain unchanged and no error gets returned. Any email that didn't yet exist is also properly created.
If you face this problem on the latest XF2 version and would like me to troubleshoot it, please send me your current list through a private conversation (you can use the Export button to generate the XML file).
Thank you for taking the time to review this resource!
EDIT: XenForo v2.0.3+ got this issue fixed :)
For more information on what caused it, see: https://xenforo.com/community/threads/banning-an-email-throws-error-when-the-administrator-attempts-to-re-add-it-by-using-a-modified-case.144074/#post-1231296