Re: FAILED: An error occured while opening a virtual disk. Verify that the Converter server and the running source machines have a network access to the source and destination ESX/ESXi hosts.

Had the same issue...

1st i changed the xml content file located here: C:\ProgramData\VMware\VMware vCenter Converter Standalone\converter-worker.xml

changed the <useSsl>true</useSsl> to<useSsl>false</useSsl>, this is under <nfc>

then went on the source server and added the primary DNS address as this was using a different one than the VCenter server

Worked 100%...