Weird Time Machine errors on Leopard Server

When I open the main GIU of the Leopard Server there's always an error pop up on the screen from Time Machine. This error states that 'a' backup malfunctioned, but all backups seem ok.

I looked through the logs and found this: (note that most backups finish ok, just some with the error below)
4/24/09 9:38:15 AM /System/Library/CoreServices/backupd[64124] Starting standard backup
4/24/09 9:38:15 AM /System/Library/CoreServices/backupd[64124] Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
4/24/09 9:38:17 AM /System/Library/CoreServices/backupd[64124] No pre-backup thinning needed: 131.4 MB requested (including padding), 187.56 GB available
4/24/09 9:38:21 AM /System/Library/CoreServices/backupd[64124] Stopping backup.
4/24/09 9:38:21 AM /System/Library/CoreServices/backupd[64124] Error: (-43) SrcErr:NO Copying /private/var/imap/user/a to /Volumes/Time Machine Backups/Backups.backupdb/Saturn/2009-04-24-093816.inProgress/83D4516C-94ED-4201-9F20-9547F53209EF/Leopard Server/private/var/imap/user
4/24/09 9:38:21 AM /System/Library/CoreServices/backupd[64124] Copied 4 files (10.7 MB) from volume Leopard Server.
4/24/09 9:38:21 AM /System/Library/CoreServices/backupd[64124] Copy stage failed with error:11
4/24/09 9:38:27 AM /System/Library/CoreServices/backupd[64124] Backup failed with error: 11

I always thought that Time Machine does not backup mail stuff, but it apparently does backup parts (useless parts) of the IMAP mail store.

The fix I'm trying is to add the /var/imap/users/ dir to the exclude list in the Time Machine preferences screen. See if this helps....

Installed new backup hardware

The 500GB backup disk in the Mac server is showing a SMART failure warning. So needs replacing. We also wanted a more solid offline backup method.

We placed a DeoNet RAID external USB2.0 next to the server. Backups will now be as followed:

• Mac server Time Machine will backup itself to the RAID device
• Mailbfr running on the Mac server will backup all the mail files to the RAID device
  
• The Mac server will share the RAID using separate partitions, one for the Linux server and one for Time Machine clients on the LAN
  
• The Linux server will backups its data to the shared RAID using rsnapshot and NFS
• Macs on the LAN will backup themselves using Time Machine via the Mac server on the RAID.
  

There are two 1TB discs in the RAID, configured as mirrored RAID 1. There are also two separate discs stored off site. Once every week one of these disks will be swapped with one in the RAID. The RAID will automatically update the disc.