Spice uses component Ids, called "Reference Designators", such as V1, V2 etc for voltages, R3, R4 etc for resistors, X1, X2 for subcircuits etc. That is, the first letter of each spice component type in the netlist MUST start with a specific letter. These types are described in the Spice3F5Manual.htm in the SuperSpice directory. SuperSpice will initially assign the correct reference designator type when placing components. If an assignment is made, for example, a Mosfet model to a transistor symbol after component placement SuperSpice will not correct the reference designator for this, and the simulation will fail. Set the reference designator manually if this is done, but make sure that no duplicate reference designators occur. As an alternative, use the menu item "Edit\renumber all reference designators".
It is advised to keep .subckt blocks numerically named, e.g. X32, rather the Xbuffer, as there are currently some unresolved issues in the XSpice engine.