Can you please spend some time in analyzing the client and server side logs , below are the location ,.
It must be the first step to start troubleshooting
VMware vCenter Converter Standalone 5.x and 6.0
To collect the client and agent logs from the Job view click Job > Export logs...
To collect the client and agent logs from the Task view click Task > Export logs...
To collect client logs from the file system of the computer that is running Converter:
C:\ProgramData\VMware\VMware vCenter Converter Standalone\logs
To collect agent logs from the computer that is being converted:
C:\ProgramData\VMware\VMware vCenter Converter Standalone Agent\logs
%ALLUSERSPROFILE%\Application Data\VMware\VMware vCenter Converter Standalone Client\logs
Note: For Linux, there are no client side logs on the source machine because they are stored on the server side.