query.addBindValue(ui->eNaziv->text());
    query.addBindValue(ui->eStanje->value());
    query.addBindValue(ui->eDatum->dateTime().toString());

result is the same: QIBaseResult::exec: Parameter mismatch, expected 0, got 3 parameters

    query.prepare("INSERT INTO MATERIJALI(NAZIV, STANJE_U_SILOSU, ZADNJA_POTROSNJA) VALUES(:naz, :sil, :pot)");
    query.bindValue(":naz", ui->eNaziv->text());
    query.bindValue(":sil", ui->eStanje->value());
    query.bindValue(":pot", ui->eDatum->dateTime().toString());

return: QIBaseResult::exec: Parameter mismatch, expected 0, got 3 parameters

It seems that QIBaseDriver don't support named placeholders. But when I try with positional placeholders, the result is the same:

    query.prepare("INSERT INTO MATERIJALI(NAZIV, STANJE_U_SILOSU, ZADNJA_POTROSNJA) VALUES(?, ?, ?)");
    query.bindValue(0, ui->eNaziv->text());
    query.bindValue(1, ui->eStanje->value());
    query.bindValue(2, ui->eDatum->dateTime().toString());

"QIBaseResult::exec: Parameter mismatch, expected 0, got 3 parameters".

When exactly is that one called ?

#include "listform.h"
#include <QVBoxLayout>
#include <QAbstractItemView>
#include <QHeaderView>
#include <QSqlRelationalDelegate>

ListForm::ListForm(QWidget *parent)
{
    QVBoxLayout *vl = new QVBoxLayout(parent);
    toolBar = new QToolBar(parent);
    tableView = new QTableView(parent);
    tableView->setSelectionBehavior(QAbstractItemView::SelectRows);
    tableView->setSelectionMode(QAbstractItemView::SingleSelection);
    tableView->resizeColumnsToContents();
    tableView->horizontalHeader()->setStretchLastSection(true);
    tableView->setItemDelegate(new QSqlRelationalDelegate);


    vl->addWidget(toolBar);
    vl->addWidget(tableView);

    this->setLayout(vl);

    actionNew = new QAction(toolBar);
    actionNew->setIcon(QIcon(":images/new.png"));

    actionEdit = new QAction(toolBar);
    actionEdit->setIcon(QIcon(":images/edit.png"));

    actionDelete = new QAction(toolBar);
    actionDelete->setIcon(QIcon(":images/delete.png"));

    actionRefresh = new QAction(toolBar);
    actionRefresh->setIcon(QIcon(":images/refresh.png"));

    toolBar->addAction(actionNew);
    toolBar->addAction(actionEdit);
    toolBar->addSeparator();
    toolBar->addAction(actionDelete);
    toolBar->addSeparator();
    toolBar->addAction(actionRefresh);
}

void ListForm::setModel(QSqlRelationalTableModel *model)
{
    tableView->setModel(model);

}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QSqlDatabase db = QSqlDatabase::addDatabase("QIBASE");
    db.setHostName("localhost");
    db.setDatabaseName("/home/vital/WMS/db/wms.fdb");
    db.setUserName("SYSDBA");
    db.setPassword("masterkey");

//    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
//    db.setHostName("localhost");
//    db.setDatabaseName("jTrade");
//    db.setUserName("root");
//    db.setPassword("Gladiator#2017");

    if (!db.open()) {
        qDebug() << "Cannot open DB!" << db.lastError();
    } else
        qDebug() << "Connect to DB!";


    QSqlRelationalTableModel *model = new QSqlRelationalTableModel(this);
    model->setTable("users");
    model->setRelation(model->fieldIndex("id_employee"),QSqlRelation("employees","id","name"));
    model->select();
    model->setEditStrategy(QSqlTableModel::OnRowChange);
    model->setSort(model->fieldIndex("name"),Qt::AscendingOrder);

    ListForm *lf = new ListForm();
    lf->setParent(this,Qt::Window);
    lf->show();
    lf->setModel(model);
}

[Added code tags ~kshegunov]

Can you show the query you are using ?