Installed Leopards 1.5.2 Update and Time Machine gives you the <irony>well known</irony> error number 11? No error number at all? Yeah, would be to easy to display some information in the error popup. Just have a look at the system logs and you'll most probably find it related to process backupd. It looks something like this:
Found it and want to fix it? You'll find the solution here. Backups sometimes can be a real pain ...