I was receiving an error “Failed to Open File” when trying to perform a P2V with the VMware Converter tool. It turned out not to actually be a file or permissions problem but a DNS resolution issue. When Virtual Center passes the information to the Converter Agent on the host being migrated it sends the host name, not the IP address of the ESX server. If the machine can’t resolve that name it will fail with the “Failed to Open File” error.
One way to fix this is with a host file. Add the vcenter server and esx host to the host file of the machine to be converted if a DNS server resolution is not an option for the client. You should always have a DNS server for your ESX environment so adding a record there if needed would work.
Hope it helps