Why can streams hang?

Question

Qt

I am writing a multi-threaded application in QT. There are two main threads: UI + Class (inherited from QThread).
The class creates a QThreadPool for 32 threads with a list of tasks (1000+) - downloading files from the Internet, unpacking them and saving them to disk. The code is checked (it worked before - before refactoring). But at some point in debug mode, sometimes the threads end with a result = 3, sometimes while Windows is running, it makes an Alert sound (like a pop-up window, but there is no window), and the application freezes, or rather these threads. Without a debugger, the application just silently crashes.
QT 5.5

Answer 1

Without a code, only psychics can find out the reason for the freezing and crashing of the application. But they are all on vacation.

Answer 2

It's not a freeze, it's a crash, it's just that under the debugger, the execution stops instead of closing.