V-79-57344-38260 - Unable to create a snapshot of the virtual machine. The virtual machine no longer exists or may be too busy to quiesce to take the snapshot.
This error is extremely generic and can be one of 10 possible problems. For a list of all possible problems see the following knowledge base article from Symantec:
The most common cause which is not documented heavily on the Internet is the lack of the Storage APIs. The Storage APIs are only available in VMware vSphere Standard Edition and higher. If your using the free version of ESXi, you will not have the Storage API and hence your backups will not work. Check the licensing page on your ESX host to see if you have the Storage API in your license key:
If you don't have it, upgrade your license key and success will be yours :).