When attempting a backup or restore the error “No such host is known” is displayed
Things to check
1.Ping the ESX and Virtual Center hosts by FQDN, Do they resolve to the correct IP address or at all?
a.If go into your hosts VIA Virtual Infrastructure client directly to your ESX hosts or though your Virtual Center, select configuration tab, then select DNS and Routing. Select properties and change your DNS information to the correct information
b.This related article from VMware may also help.
2.You may have to add in a host record in your DNS servers as well
3.Alternatively you can add in a host file entry that resolves to your FQDN displayed in your ESX environment.
4.If you are using nothing but IP addresses in your environment, you will need to add in DNS names in your ESX environment.
5.If the name of your environment is shows the short name VS the FQDN see the article below.