Had issue this week come up where vCloud Director is unable to communicate with vCenter server. I had this many years ago but seems to be still relevant in vCD-SP8 so thought just write up about it with reference to good information:
In my environment this caused template deployments to fail with "Cannot retrieve list of the supported Operating Systems from Provider VDC "PVDC". Please ensure vCenter is connected and available."
After reviewing vCenter Server vSphere resource under Manage and Monitor I found that vCenter is not connected.
KB 1035506 from VMware documents this issue but request that you call support to get this resolved.
The issue seems to be related to quart table and I am not going to get into much details here since Jason Boche wrote a nice detailed article how the problem comes about and how to resolve it.