pthreadpool | Portable thread pool for C/C++ | Architecture library

I have the following code which I would like to convert from using ThreadPool to use of ProcessPoolExecutor since it is all CPU intensive calculations and when i observe the CPU monitor I note that my 8 core processor is only using a single thread.

I have written an application with a main window that opens multiple sub windows as child windows (WS_CHILD) - i.e. a statusline, a bar with buttons and a content part. The main window so far is a frame for the others and acts in background. It is also organizing messages and many more. The content part can be replaced by other windows with other content. Every window has its own thread that works some kind of programming language that fills the window with its content. To speed up the change of the program, I use a thread pool. So if a window is closed, it releases the thread to suspend mode and the next window that is opened can use this thread again.

So far, so good. This construction works fine for more than 10 years and is the base for several programs.

Now with the latest upgrade of Windows 10 20H1 the program stopped working. It looks like the message queue within windows is working for a while and then hangs up in DispatchMessage after some seconds. Some windows do not receive any more messages in the WndProc although the underlying thread is just waiting for work. The fastest method to reach this, is to open a window not with WS_CHILD but with WS_POPUP. Then the window occurs but it does not receive a paint message and never get filled. It is just the frame After this, the program does not act any longer.

I have tried to find PostMessage and SendMessage that is sent from one thread to another. There had been a view, but they had been eliminated now (and did not make any problem in the past).

Another idea was to remove the thread pool and use a new thread for every new window. But this did not make anything better. The only thing that helped was to keep open all created windows. As the programs often run the whole day or even several days this is no solution either. I cannot keep some hundreds of windows in background.

Any ideas what could be the problem? Or does someone have similar experiences with the 20H1 upgrade of windows?

Unfortunately the programs on this base are running on many computers all over the world and some customers already made the upgrade of Windows. The quick solution was to roll back the upgrade but this cannot be the eternal solution of course.

EDIT: Here I have a sample for the problem:

SampleThreadWindow.cpp

I'm trying to use vorbrodt's thread-pool implementation in my code: https://vorbrodt.blog/2019/02/27/advanced-thread-pool/

So, I have a function with this signature:

I'm, trying to use CPTL thread-pool for my application.

So, i have a function with this definition: