Periodic (daily) Time Machine failures

So every time this happens on my Mac, this error is logged in the console:

9/10/09 12:35:59 PM /System/Library/CoreServices/backupd[4092] Starting standard backup 
9/10/09 12:36:02 PM /System/Library/CoreServices/backupd[4092] FSMountServerVolumeSync failed with error: -6600 for url: afp://[email protected]/ReadyNAS 
9/10/09 12:36:02 PM /System/Library/CoreServices/backupd[4092] Authentication error - the correct user or password info may not exist in the System.keychain or the server may no longer allow access for this user. 
9/10/09 12:36:02 PM /System/Library/CoreServices/backupd[4092] Authentication error - the correct user or password info may not exist in the System.keychain or the server may no longer allow access for this user. 
9/10/09 12:36:07 PM /System/Library/CoreServices/backupd[4092] Backup failed with error: 29 

Then, the backup proceeds normally the next time Time Machine is fired up automatically:

9/10/09 1:36:00 PM /System/Library/CoreServices/backupd[4092] Starting standard backup 
9/10/09 1:36:02 PM /System/Library/CoreServices/backupd[4092] Mounted network destination using URL: afp://[email protected]/ReadyNAS 
9/10/09 1:36:02 PM /System/Library/CoreServices/backupd[4092] Backup destination mounted at path: /Volumes/ReadyNAS 
9/10/09 1:36:05 PM /System/Library/CoreServices/backupd[4092] Disk image /Volumes/ReadyNAS/Egwene_001d4f49f3a4.sparsebundle mounted at: /Volumes/Backup of Egwene 
9/10/09 1:36:05 PM /System/Library/CoreServices/backupd[4092] Backing up to: /Volumes/Backup of Egwene/Backups.backupdb 
9/10/09 1:36:16 PM /System/Library/CoreServices/backupd[4092] No pre-backup thinning needed: 1.44 GB requested (including padding), 1.11 TB available 
9/10/09 1:36:16 PM /System/Library/CoreServices/backupd[4092] Waiting for index to be ready (906 > 0) 
9/10/09 1:36:55 PM /System/Library/CoreServices/backupd[4092] Copied 1 files (6 KB) from volume Storage. 
9/10/09 1:37:34 PM /System/Library/CoreServices/backupd[4092] Copied 882 files (6.8 MB) from volume Egwene. 
9/10/09 1:37:38 PM /System/Library/CoreServices/backupd[4092] Copied 887 files (6.8 MB) from volume StorageRAID. 
9/10/09 1:37:38 PM /System/Library/CoreServices/backupd[4092] No pre-backup thinning needed: 1.43 GB requested (including padding), 1.11 TB available 
9/10/09 1:37:42 PM /System/Library/CoreServices/backupd[4092] Copied 1 files (6 KB) from volume Storage. 
9/10/09 1:37:53 PM /System/Library/CoreServices/backupd[4092] Copied 425 files (6 KB) from volume Egwene. 
9/10/09 1:37:57 PM /System/Library/CoreServices/backupd[4092] Copied 430 files (6 KB) from volume StorageRAID. 
9/10/09 1:38:00 PM /System/Library/CoreServices/backupd[4092] Starting post-backup thinning 
9/10/09 1:38:10 PM /System/Library/CoreServices/backupd[4092] Deleted backup /Volumes/Backup of Egwene/Backups.backupdb/Egwene/2009-09-09-132812: 1.11 TB now available 
9/10/09 1:38:19 PM /System/Library/CoreServices/backupd[4092] Deleted backup /Volumes/Backup of Egwene/Backups.backupdb/Egwene/2009-09-09-122809: 1.11 TB now available 
9/10/09 1:38:19 PM /System/Library/CoreServices/backupd[4092] Post-back up thinning complete: 2 expired backups removed 
9/10/09 1:38:19 PM /System/Library/CoreServices/backupd[4092] Backup completed successfully. 
9/10/09 1:38:22 PM /System/Library/CoreServices/backupd[4092] Ejected Time Machine disk image. 
9/10/09 1:38:22 PM /System/Library/CoreServices/backupd[4092] Ejected Time Machine network volume. 

Is this helpful to know? Error -6600 isn't terribly helpful, nothing much turns up when searching for it (aside from 'Yeah, that's an authentication error.').

Any ideas? This happens to me about once a day, sometimes twice.

Thanks!

\marc