The fix here is using the command line utilities to explicitly re-associate your local disk's TimeMachine backups with the NAS TimeMachine partition on the far end (best I can tell). If you're fairly sure you don't have network issues, then I'd be curious what would cause your Macbook to disassociate. Has it been working since you set it up again? What's the timeline for this change? And what changes have occurred locally on the Macbook? Update to Mavericks, etc.?

Hrm. Wasn't there an issue you had a while ago where Windows file sharing was considerably faster than Apple file sharing? The suggested steps have you using AFP to perform time machine backups.