NAV 2017 Page webservice error "401 - Not authorized"

Attempt to consume page webservice via code:
Customers_Service customersservice = new Customers_Service { UseDefaultCredentials = false, Credentials = new NetworkCredential("xxx", "*****", "domain") };
List<Customers_Filter> customersfilterarray = new List<Customers_Filter>();
Customers_Filter f1 = new Customers_Filter { Field = Customers_Fields.No, Criteria = balanceop.PartnerNo }; //e.g. 'C22*'
customersfilterarray.Add(f1);
Customers[] c = customersservice.ReadMultiple(customersfilterarray.ToArray(), null, 100); //produces not authorized error
- user, password and domain are valid Windows login with SUPER permission
- IIS windows login is enabled

Any suggestion will help