I've managed to use some simplified variant of your code with Qt 4.8.3 and MSVC 2010:
ui.tableWidget->setColumnCount(5);
ui.tableWidget->setRowCount(5);
char *memChunck = new char[25];
for ( int i = 0; i < 25; ++i ) {
memChunck[i] = i + 65;
}
for ( int i = 0; i < 25; ++i ) {
QTableWidgetItem *item = new QTableWidgetItem(QString::number( memChunck[i], 16).toUpper() );
ui.tableWidget->setItem(i / 5, i % 5, item);
}