Error -54 on idisk backup

[zaconline]

Occasionally I get a back failed when backing up to idisk using backup. The error message is -54.
Connected to .Mac
Begin backup at 2008-05-12 04:32:47.636 -0700
Scanning files...
Ignoring file /Users/zaciMac/Library/Acrobat User Data/8.0_x86/Synchronizer/Commands: Invalid type.
Ignoring file /Users/zaciMac/Library/Acrobat User
Data/8.0_x86/Synchronizer/Notification: Invalid type.
Acquired a destination for the backup.
Unable to copy /Users/zaciMac/Documents/Microsoft User Data/Office 2004 Identities/Main Identity/Database: Mac OS error -54.
End backup at 2008-05-12 04:33:57.991 -0700

Sometimes I get it several nights in a row and then it will be fine for days.

What might be causing this?

[VirtualTracy]

I found this at the Apple website:

Troubleshooting iDisk sync issues

There's also this where you can find a contact form at the bottom of the page.