Although you were assisted by our Chat Support, I did want to share our solution here in the Community as well. This upload experience seemed to be caused from columns in your file. Deleting all columns except your email address column allowed it to upload properly. In cases where this has happened for other users, we would even suggest renaming any custom fields to something other than "Unmatched." Even matching all file headers to existing custom fields is a workaround that works as well.