It is rather hard to diagnose without logs. And as a convenience, the Converter Standalone not only produces plenty of them by default, but also offers an option to export them via the GUI. Also, it is always helpful to include software versions of the used products.
Working with what you've provided, the error apparently occured in the phase of reconfiguration of the virtualized system. Apparently, the Converter was unable to access the system volume. You could retry the conversion but choose a different type of storage controller (BusLogic) in order to work around a possible Converter bug. Also, make sure that the storage controller driver is present on your machine before conversion. Read KB 1005208 for further instructions.