When using VMware vConverter Standalone 6.1 or newer, performing a P2V may fail very early in the process with the following error:
You can work around the issue by enabling Use proxy mode on the Destination System page at the conversion wizard. As an alternative workaround, perform the P2V directly to a ESX host.
The reason for the error has to do with the ESX host's certificate not being trusted when performing a P2V to a vCenter.
If the Use proxy mode does not resolve your issue, please check whether the FQDN of the ESX host is resolvable on the machine running vConverter. Even when using strictly IP addresses, the FQDN still needs to resolve (at least for version 6.1).