@ulix
You have a QTimer in a thread which does not run the Qt event loop, does it? So no signals will be delivered. Hence your finding:

The timer expires but the slot never gets called, by the way if a I call QCoreApplication::processEvents(); after the sleep time the slot gets called.

That processEvents() is the first time the evet loop is invoked. Your thread needs to be running the event loop all the time if that's where the timer is ticking (unless you move the timer to the main UI thread). See QThread::run() & QThread::exec().

OK, it was a bit premature to ask immediately.

I found this post https://forum.qt.io/topic/54438/solved-qt-5-4-1-qtimer-using-up-all-handles-for-window-manager-objects

Actually with QNetworkAccessManager I was a bit sloppy. Need to check if it helps already.

Edit[koahnig]: It helped. Therefore, pay attention when generating new QNetworkAccessManger objects.