Contact client takes long time in the backup

Is this the first time you are backing up? I would suggest first test network using ping etc. Update /etc/hosts, update aliases on the client resource. Thentest manual backup form the client, use save -s. If this works, test scheduled backup form server. Till this time backup only file system.

If & when all goes well, test NMD backups.

The backup worked fine before, just in recent several days it has the problem in contacting the client. I used the ping test and it pings successfully to the client. Should I restart the client machine?

You dont need to restart the machine. Just restart the service. Also check connectivity with nslookup.

I have restarted the backup services on the client, and the server. But I was not able to login to the management console after restarting the service on the server. So, I restarted the server host. Also, I realized that there was not enough space to do the backup, so I free up some more disk space from the old data that no longer needed(by changing the mode of the volume from full-appendable to full-recycle). Then I restart the backup task. By looking at the Monitor/Device log, I can see it shows some progress in writing the data to the backup tape. Thanks for your helps!!!!