When IPv6 is enabled on one or more network interfaces in a Windows proxy, virtual machine backup can fail while attempting to open a disk with the proxy. The cause is a stack buffer overrun in the libcurl.dll when trying to enumerate DNS servers having multiple IPv6 network interfaces.
Ports required for proper VDDK API communication may be blocked between core server and vCenter. In this case, it may be port 902. Please review port requirements in this KB article - Complete List of Ports Used by Rapid Recovery.