It looks like an NFC (the protocol converter uses for transferring data) issue at first glance, and these are hard to debug.
I wonder whether it is possible to convert the machine with just the C volume and then copy the other one a different way (SMB, FTP, ..)?
As for the locked file - if it is locked by converter (the client side) , it should be the vstor driver (vstor2-mntapi20-shared); restarting the worker service might not be enough. But it might also be locked by the server side. If so, try restarting the vpxa service on the destination ESX.
Regards
Plamen