"No such file or directory" error.

[TheDreadPirate]

Windows and, I'm assuming, your WD NAS could possibly have sleep modes or low power states that drops connectivity.

Could also be just general network flakey-ness. But it should be re-establishing connection and re-add content when the connection is restored and the library scan job runs.

I don't have enough information to say for certain that is the cause. Nor enough knowledge about how you have things setup to guide you beyond those vague suggestions.