>However, when I start to create a backup, the process would terminate with an error :-(<
That is not correct. According to your post, you got to 66% before it terminated with an error.
I have the same problem. In my case, I make a separate virtual disk, and mount it with 2003 server to project an NFS volume. I use GhettoVCB, as you do. It uses the standard vkmfstools in the script to do the clone/backup. I've proven that the problem also happens when you use vkmfstools from the command line. GhettoVCB has a nasty habit of hiding from you the error code. The error code is that it ran out of disk space, which we both know didn't happen. Once the problem occurs, the only way I have been able to fix it is to delete the virtual disk, and start over, however, the problem will return. The problem didn't surface until I moved from ESXi 4.0 to 5.1 on the servers. Through this process, the hardware version off the VMs did not change, nor would I dare, since I may need to throw out 5.1 and move back to 4.x.