Heap Corruption while using CreateWindowExW

I have some problems with heap corruption. The warning can be observed while using CreateWindowExW function. I know that it is usually a memory error, but how could I search it out in such situation? There are no new variables before calling CreateWindowExW and I can't step into this function. Here is the code.

HWND GetMainWnd(HINSTANCE hInstance){
static HWND hWnd = NULL;
if (hWnd)
    return hWnd;

RETURN_AT_ERROR(hInstance, NULL);

WNDCLASSEX wcex = { sizeof(WNDCLASSEX) };
wcex.style          = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc    = MainWndProc;
wcex.hInstance      = hInstance;
wcex.hCursor        = ::LoadCursorW(NULL, IDC_ARROW);
wcex.lpszClassName  = g_config->GetWndClass();

ATOM atom = ::RegisterClassExW(&wcex);
RETURN_AT_ERROR(atom != 0, NULL);

hWnd = ::CreateWindowExW(WS_EX_LEFT, g_config->GetWndClass(), 0, WS_POPUP | WS_MINIMIZEBOX | WS_CLIPCHILDREN, 0, 0, 0, 0, 0, 0, hInstance, 0);

return hWnd;}

On this string

hWnd = ::CreateWindowExW(WS_EX_LEFT, g_config->GetWndClass(), 0, WS_POPUP | WS_MINIMIZEBOX | WS_CLIPCHILDREN, 0, 0, 0, 0, 0, 0, hInstance, 0);

there is a warning message box

Windows has triggered a breakpoint in drm.exe. This may be due to a corruption of the heap, which indicates a bug in drm.exe or any of the DLLs it has loaded. This may also be due to the user pressing F12 while drm.exe has focus. The output window may have more diagnostic information.

I press "Continue" and it shows

Unhandled exception at 0x77dae753 in app.exe: 0xC0000374: A heap has been corrupted.

However CreateWindowExW returns a non-zero value and window is created as it should be...

As pointed out above, heap corruption is often detected after the real corruption has already occurred by some DLL/module loaded within your process. From your post it looks like this issue is windows platform specific so I would suggest you to use WinDBG/Pageheap and find out where actual memory corruption is happening. One very very good article about heap memory corruption analysis can be found from the book "Advanced Windows Debugging, Author: By: Mario Hewardt; Daniel Pravat" Chapter 06

http://advancedwindowsdebugging.com/ch06.pdf