Random Crashes in QNetworkAccessHttpBackend::postRequest()

We're getting totally random crashes in QNetworkAccessHttpBackend::postRequest() , and I didn't found any closed bug reports here.

This is a part of a crash report:
4 libc++abi.dylib 0x9a3c32bf __cxa_pure_virtual + 26
5 QtCore 0x02de59a1 QCoreApplication::postEvent(QObject*, QEvent*, int) + 331
6 QtCore 0x02de5a10 QCoreApplication::postEvent(QObject*, QEvent*) + 32
7 QtCore 0x02dfab43 QMetaObject::activate(QObject*, QMetaObject const*, int, void**) + 3013
8 QtNetwork 0x035fe649 QNetworkAccessHttpBackend::startHttpRequest() + 49
9 QtNetwork 0x03582d6d QNetworkAccessHttpBackend::postRequest() + 7483
10 QtNetwork 0x0357711d QNetworkAccessBackend::start() + 821
11 QtNetwork 0x0358ebcb QNetworkReplyImplPrivate::_q_startOperation() + 363
12 QtNetwork 0x03592c66 QNetworkReplyImplPrivate::setup(QNetworkAccessManager::Operation, QNetworkRequest const&, QIODevice*) + 1886
13 QtNetwork 0x035739f3 QNetworkAccessManager::createRequest(QNetworkAccessManager::Operation, QNetworkRequest const&, QIODevice*) + 2051
14 QtNetwork 0x03572c8f QNetworkAccessManager::get(QNetworkRequest const&) + 49

It seems like something is wrong with the delegate. Is this a known bug?

We've found several other crashes in the net:
https://retrace.fedoraproject.org/faf/reports/37996/
https://bugs.kde.org/show_bug.cgi?format=multiple&id=292965
https://bugs.kde.org/show_bug.cgi?id=304376
https://bugs.kde.org/show_bug.cgi?id=301828
http://old.nabble.com/-Bug-292965--New%3A-Taskbar-freezes-%28no-response%29-p33236710.html
http://www.kadu.im/redmine/issues/2643
http://stackoverflow.com/questions/10974247/qnetworkaccessmanagerget-cause-pure-virtual-method-called

Unfortunately, it is not possible to reproduce this crash on any of your computers.