When you are running the BE job when you are getting the snapshot errror, just check what is the error that is shown in VSphere client ui ? It should show something Createsnapshot failed because of ....
Is the virtual machine being backed up using BE...
The agent on the guest machien is requied if you wish to do GRT , if you do not want to do GRT then we do not require the agent inside the guest.
For the virtual machine where you are getting the error regarding unbale to create the snapshot, try to ...
Can you check this
http://www.symantec.com/business/support/index?page=content&id=TECH124681
If feasible, can you post the error code you are getting .
Mohit