>However, when I start to create a backup, the process would terminate with an error :-(<In my case, I make a separate virtual disk for a 2003 server, and use it to project an NFS volume.
That is not correct. You got to 66%.
I have the same problem. In my case, I make a separate virtual disk for a 2003 server, and use it to project an NFS volume. It works for the first few backups, and then has the problem. That has not always been the case. The problem didn't surface until I went from ESXi 4.0 to 5.1. It appears it is a 5.1 issue.