NAServerSolverParams is an abstract type. When creating an instance of it as SolverParams, you need to specify the type of NAServerSolverParams that you want to use.
Your request starts like this
<soapenv:Body>
<ns:Solve>
<SolverParams>
but it should be this instead
<soapenv:Body>
<ns:Solve>
<SolverParams xsi:type="NAServerODCostMatrixParams">