backup failed error message

You may find answers here: Pondini's Time Machine FAQ.

First, power cycle the entire network as follows:

Power off every network device.....modem, routers, computers, printers, etc.....in any order that you want

Wait a few minutes

Power up the modem and let it run 2-3 minutes

Power up the next device connected to the modem and let it run a full minute

Keep powering up devices one at a time about a minute apart until the entire network is back up

Wait a few minutes, then try a backup

If no luck, check this document:

http://pondini.org/TM/Troubleshooting.html

Match the error that you see with the list of error messages that you see in C1-C19