The problem is QNetworkAccessManagerPrivate::findBackend() returns NULL because factoryDataShutdown = true.

factoryDataShutdown is set to true due a bug.

This is the relevant code in qnetworkaccessbackend.cpp:

static bool factoryDataShutdown = false;
class QNetworkAccessBackendFactoryData: public QList<QNetworkAccessBackendFactory *>
{
public:
QNetworkAccessBackendFactoryData() : mutex(QMutex::Recursive) { }
~QNetworkAccessBackendFactoryData()

{ QMutexLocker locker(&mutex); // why do we need to lock? factoryDataShutdown = true; }

QMutex mutex;
};
Q_GLOBAL_STATIC(QNetworkAccessBackendFactoryData, factoryData)

So, when a QNetworkAccessBackendFactoryData object is deleted, it sets factoryDataShutdown to true.

Now here is the code for Q_GLOBAL_STATIC():

#define Q_GLOBAL_STATIC(TYPE, NAME) \
Q_GLOBAL_STATIC_INIT(TYPE, NAME); \
static TYPE *NAME() \
{ \
if (!this_##NAME.pointer && !this_##NAME.destroyed)

{ \ TYPE *x = new TYPE; \ if (!this_##NAME.pointer.testAndSetOrdered(0, x)) \ delete x; \ else \ static QGlobalStaticDeleter<TYPE > cleanup(this_##NAME); \ }

\
return this_##NAME.pointer; \
}

So if 2 threads try to create the pointer at the same time, one of them will call "delete x".

In the case of QNetworkAccessBackendFactoryData() this disables the backend factory (on program startup) by setting factoryDataShutdown = true.