You'll probably want this posted here: http://communities.vmware.com/community/vmtn/developer/forums/vcloudsdkjava
General API questions go here: http://communities.vmware.com/community/vmtn/developer/forums/vcloudapi
"- A general system error occurred: The parent virtual disk has been modified since the child was created. Parent virutal disk's content ID does not match with the parent content ID in the child"
This error message is not good in a general sense. If this message happens in the vCloud Director GUI, then there is something bigger going on. If it's exclusive to the API call for a hot clone + fast provisioning .. then there at least it narrows it down.
The error message above should only occur when fast provisioning is enabled, or snapshots are being taken (possible for hot clone of a VM).