You do not provide enough information about the conversion.
If you doing conversion through Agent you need to share converte-agent log, if you converting 'Local machine' then you need to share converter-worker log. You can find the logs in folder:
XP, 2003
%ALLUSERSPROFILE%\Application Data\VMware\VMware vCenter Converter Standalone[Agent]\logs
Vista+
%ALLUSERSPROFILE%\VMware\VMware vCenter Converter Standalone[Agent]\logs