Hi,
While, I am still wondering on your question, the error thrown seems to be " description = "The operation experienced a network error",
I will come to it later after clearing my question.
Conversion is used for transforming physical to virtual and vice versa and from workstation to esx or from other virtual products to esx like xen. I am wondering what kind of "conversion" you are talking about, since you are dealing with a movement of virtual machine in 4.0 to 5.1. Please correct me if I am wrong, and could you provide a little more details on the same.
Thanks,
zXi