one to one relationship - foreign key error PLEASE!!

 Hello :) I have a problem for a couple of weeks and I really don't know how to fix it. I have 2 tables

clienti(id_client(pk),nume, prenume, id_utilizator(fk)) and 
utilizator(id_utilizator(pk), email, id_client(fk))

 When i'm trying to add a new record I keep getting "The UPDATE statement conflicted with the Foreign Key constraint "FK_client_utilizator etc". Does anybody know how to fix it? PLEASE it drives me nuts :(. 
        I've set for id_utilizator(fk) from clienti a DEFAULT VALUE that is found on the PK domain in utilizator. I can add client information, but of course id_utilizator is the DEFAULT VALUE. When I'm trying to update it,  I get that error. Here's my code:

//adding information in client
string tip_client = null;
                    string tip_persoana = null;
                    string judet = null;
                        if (tipClientCb.SelectedItem.ToString() == "Personal")
                            tip_client = "personal";
                        else
                            if (tipClientCb.SelectedItem.ToString() == "Altul")
                                tip_client = "altul";


                        if (tipPersCb.SelectedItem.ToString() == "Persoana fizica")
                            tip_persoana = "persoana fizica";
                        else
                            if (tipPersCb.SelectedItem.ToString() == "Persoana juridica")
                                tip_persoana = "persoana juridica";


                        judet = judetCb.SelectedItem.ToString();


                        string insertClient = "INSERT INTO client (tip_client,tip_persoana,cuicnp,denumire,telefon,localitate,judet) VALUES ('"
                             + tip_client + "','" + tip_persoana + "','" + codClientTxt.Text + "','" + denumireTxt.Text + "','" + telefonTxt.Text + "','" + localitateTxt.Text+"','" + judet + "')";
                        System.Data.SqlClient.SqlCommand executaInsertClient = new System.Data.SqlClient.SqlCommand(insertClient, conn);
                        executaInsertClient.ExecuteNonQuery();
                    }
                       
                    //saving id_client
                    int id_client;
                    string selectIdClient = "SELECT * FROM client WHERE  tip_client='" + tipClientCb.Text +
                       "' AND tip_persoana='" + tipPersCb.Text +
                       "' AND cuicnp='" + codClientTxt.Text +
                       "' AND denumire='" + denumireTxt.Text +
                       "' AND localitate='" + localitateTxt.Text +
                       "' AND judet='" + judet + "'";
                    System.Data.SqlClient.SqlCommand executaSelectIdClient = new System.Data.SqlClient.SqlCommand(selectIdClient, conn);
                    System.Data.SqlClient.SqlDataReader rdr_id_client = executaSelectIdClient.ExecuteReader();
                    rdr_id_client.Read();
                    id_client = Convert.ToInt32(rdr_id_client["id_client"]);
                    rdr_id_client.Close();


                    //adding information in utilizator table
                    string insertUtilizator = "INSERT INTO utilizator (id_utilizator, id_client, parola,[e-mail]) VALUES ('"
                    + utilizatorTxt.Text + "','" + id_client + "','" + parolaTxt.Text + "','" + emailTxt.Text + "')";
                    System.Data.SqlClient.SqlCommand executaInsertUtilizator = new System.Data.SqlClient.SqlCommand(insertUtilizator, conn);
                    executaInsertUtilizator.ExecuteNonQuery();




                    //updating id_utilizator
                    string updateIdUtilizator = "UPDATE client SET id_utilizator='" + utilizatorTxt.Text + " WHERE id_client = " + id_client + "'";
                    System.Data.SqlClient.SqlCommand executaUpdateIdUtilizator = new System.Data.SqlClient.SqlCommand(updateIdUtilizator, conn);
                    executaUpdateIdUtilizator.ExecuteNonQuery();