A http response code of 503 means there’s a problem on the server. In this case, the server is your Tablo, but before you tell me your Tablo is working fine (of course it is), you need to look at why your PC can’t “see” your Tablo on your local LAN. That’s what is really going on.
TabloRipper has two way of connecting to your Tablo. One is to ask Nuvyyo for a list of Tablos you own, the other is “local” and uses your local/internal Tablo IP address.
So the general solution for your problem is to double check how your LAN is configured. Maybe even assign a static IP address to your Tablo (using your router to assign one), then telling TabloRipper what it is.
But if all else fails, enable logging in TabloRipper and use it to send me your log file using a PM. Maybe I can be more specific once I see it.
Good luck.