13. Error in model. Please verify that all cells and constraints are valid.

This message means that the internal "model" (information about the adjustable cells, target cell, constraints, Solver options, etc.) which is created by the SOLVER.12A add-in and passed to SOLVE123.DLL is not in a valid form. You might receive this message if you are using the wrong version of either SOLVER.12A or SOLVE123.DLL, or if you have modified cells on the "hidden sheet" used by the Solver, either interactively or in a LotusScript or macro program.