Move the following
//Ingelogd
kService = new KlantService();
ingelogdeKlant = kService.GetAllByUsername(inlogsessie.GetUsername());
Above the if (!Page.IsPostBack)
Your ingelogdeKlant
is only instantiated on the first Page_Load
and not on subsequent postbacks.
Your Page class is instantiated each time a request (PostBack or not) is made, any members need to be initialised each time they are needed; they won't persist because the Page object doesn't persist.