I am really surprised I haven't run into this before... but I guess there is a weird bug with many copiers out there that prevents scan to folder from working correctly under the following circumstances:
1) The source copier is using domain authentication
2) The source copier is trying to scan to a folder
3) The destination shared folder is on a Windows 2008 or 2008 R2 server
4) The destination server is also a domain controller
5) The destination shared folder is shared using 'authenticated users' as it's share permission, rather than specific users or the 'Everyone' group.
When these conditions are met, the scanner seems unable to scan to a folder. I found the two fixes so far:
1) Change the 'share' level permissions to 'Everyone' and 'full access'. This allows the copier to send files without authentication, thus bypassing the problem.
2) Change the user specified in the copier as the authentication account from a DOMAIN\Username or just 'Username' to their UPN format... Username@domain.suffix.
Either of those two changes seem to do the trick!