I'm not sure if it's a proper way but removing scrollAreaWidgetContents directly from .ui file helps as well.

Modifying an ui file manually is never a good idea and for sure no solution.

    <item>
     <widget class="ImageScrollArea" name="imageScrollArea">
      <property name="widgetResizable">
       <bool>true</bool>
      </property>
      <widget class="QWidget" name="scrollAreaWidgetContents">
       <property name="geometry">
        <rect>
         <x>0</x>
         <y>0</y>
         <width>780</width>
         <height>538</height>
        </rect>
       </property>
      </widget>
     </widget>
    </item>

imageScrollArea->setWidget(scrollAreaWidgetContents);

When creating it via designer a default widget is already said to the scroll area. Add you label to this instead calling setWidget()

Okay, I got it. In ui_MainWindow.h there's:

imageScrollArea->setWidget(scrollAreaWidgetContents);

Is there a way to get rid of scrollAreaWidgetContents? Am I using QScrollArea in a wrong way?

And you're sure you call setWidget() only once?

Yup, calling setWidget() only in my custom ScrollArea's constructor.

Please remove all other stuff to try to create a small reproducable example.

https://mega.nz/file/owtigShT#1M63GDHVPDqzct7i47UV5y1lRxJWjGe-lfGJwYqqgw4

1  QLabel::setPixmap(QPixmap const&)                                                                                                                                                                      0x10d252c  
2  ImageScrollArea::SetImage                                                                                                                                                      ImageScrollArea.cpp 18  0x401668   
3  MainWindow::<lambda()>::operator()(void) const                                                                                                                                 MainWindow.cpp      14  0x401850   
4  QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MainWindow::MainWindow(QWidget *)::<lambda()>>::call(MainWindow::<lambda()> &, void * *)             qobjectdefs_impl.h  146 0x401cb8   
5  QtPrivate::Functor<MainWindow::MainWindow(QWidget *)::<lambda()>, 0>::call<QtPrivate::List<>, void>(MainWindow::<lambda()> &, void *, void * *)                                qobjectdefs_impl.h  256 0x401c98   
6  QtPrivate::QFunctorSlotObject<MainWindow::MainWindow(QWidget *)::<lambda()>, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void * *, bool *) qobjectdefs_impl.h  443 0x401c69   
7  void doActivate<false>(QObject *, int, void * *)                                                                                                                                                       0x68b99915 
8  QAction::activate(QAction::ActionEvent)                                                                                                                                                                0xfb3df3   
9  QMenuPrivate::activateCausedStack(QVector<QPointer<QWidget>> const&, QAction *, QAction::ActionEvent, bool)                                                                                            0x110c4eb  
10 QMenuPrivate::activateAction(QAction *, QAction::ActionEvent, bool)                                                                                                                                    0x1112572  
11 QMenu::mouseReleaseEvent(QMouseEvent *)                                                                                                                                                                0x1113459  
12 QWidget::event(QEvent *)                                                                                                                                                                               0xff3d78   
13 QMenu::event(QEvent *)                                                                                                                                                                                 0x111564e  
14 QApplicationPrivate::notify_helper(QObject *, QEvent *)                                                                                                                                                0xfb790e   
15 QApplication::notify(QObject *, QEvent *)                                                                                                                                                              0xfbf1ea   
16 QCoreApplication::sendSpontaneousEvent(QObject *, QEvent *)                                                                                                                                            0x68a65588 
17 QApplicationPrivate::sendMouseEvent(QWidget *, QMouseEvent *, QWidget *, QWidget *, QWidget * *, QPointer<QWidget>&, bool, bool)                                                                       0xfbd83b   
18 QWidgetWindow::handleMouseEvent(QMouseEvent *)                                                                                                                                                         0x100a37e  
19 QWidgetWindow::event(QEvent *)                                                                                                                                                                         0x100cc06  
20 QApplicationPrivate::notify_helper(QObject *, QEvent *)                                                                                                                                                0xfb790e   
21 QApplication::notify(QObject *, QEvent *)                                                                                                                                                              0xfbe3e3   
22 QCoreApplication::sendSpontaneousEvent(QObject *, QEvent *)                                                                                                                                            0x68a65588 
23 QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent *)                                                                                                                 0x61979ff4 
24 QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent *)                                                                                                   0x6197b315 
25 QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                                                  0x6195726c 
26 QEventDispatcherWin32::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                                                            0x68abb6c0 
27 QWindowsGuiEventDispatcher::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                                                       0x6a903e85 
28 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                                                                                0x68a63a15 
29 QCoreApplication::exec()                                                                                                                                                                               0x68a6cd75 
30 qMain                                                                                                                                                                          main.cpp            10  0x4016e0   
31 WinMain                                                                                                                                                                        qtmain_win.cpp      97  0x40390d   
32 __tmainCRTStartup                                                                                                                                                                                      0x4013c7   
33 WinMainCRTStartup                                                                                                                                                                                      0x4014cb   

Also I found "Create Full Backtrace" option. Is it needed?

post the backtrace maybe?

I'm sorry, that's the part I don't understand the most. I don't get what you mean by backtrace and where I can see it in qtc.
Is call stack (the screen I sent before) not the same as backtrace?

And what's the backtrace when it crashes?

How do I check that?

MainWindow.cpp

MainWindow::MainWindow(QWidget* parent)
	: QMainWindow(parent)
	, m_ui(std::make_unique<Ui::MainWindow>())
{
	m_ui->setupUi(this);

	connect(m_ui->actionOpen, &QAction::triggered, this, [this]() {
		auto filename = QFileDialog::getOpenFileName(this, "Open", "", "Images (*.png *.jpg)");
		m_ui->imageScrollArea->SetImage(QPixmap(filename)); // <-- SetImage call is here
	});
}

MainWindow::~MainWindow()
{
}

Also what exactly do you mean by backtrace?
Call stack is fine?

Any idea why?

From where do you call SetImage? Please also show the backtrace.