windows callback function freezes with any new callback event - windows

Question

I wrote a Win32 backup program, that runs fine - however it will hang as soon as any callback event acours.
The function calls in case of IDC_F_BACKUP and IDC_F_SAVEAS work ok, but whenever the input and output folders
are defined and 'start' (IDC START) is hit, any other callback event (for example a mouse click or a window move)
will freeze this application (and it's icon changes to a windoews default one).
As I understood, a callback function should return within a short timeframe. I have no idea how to realize this
requirement since whenever the 'start' button has been hit the backup runs for much more than 30 minutes.
Here my windows procedure
LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
/* ========================================================================
** Callback loop for the Windows procedure
** ========================================================================
*/
{
switch(msg) {
//case WM_MOVE:
//case WM_SHOWWINDOW:
//case WM_WINDOWPOSCHANGING:
case WM_PAINT:
case WM_CREATE:
DisplaySettings();
ActivityMonitor(L"\0");
break;
case WM_CLOSE:
DestroyWindow(hWnd);
DestroyWindow(hWnd);
case WM_DESTROY:
PostQuitMessage(0);
break;
case WM_COMMAND: {
switch(LOWORD(wParam)) {
case IDC_START: {
DWORD p;
const WCHAR szEndMsg[]=L"====> backup successfully completed <====";
if((szInpPath[0]==L'?') || (szOutpPath[0]==L'?')) {
MessageBoxW( NULL, L"Path not found", L"Error",MB_OK );
}else {
// create and initialize error log file:
strCopyW(&szOutpPath[nOP_End], (WCHAR*)L"\\skipped_files.log", MAX_PATH);
//MessageBoxW( NULL, szOutpPath, L"Log file", MB_OK );
hLog = _wfopen(szOutpPath, L"a+");
Test((hLog!=0), L"can not create log file");
szOutpPath[nOP_End] = L'\0';
fwprintf(hLog, L"\n\n\n");
for(p=0; p<20; p++) fwprintf(hLog, L"----");
fwprintf(hLog, L"\nBackup starting at %s", szInpPath);
fwprintf(hLog, L"\nFollowing files could not be archived:\n");
// go ...
BackupDirectory(); // <========== this function runs those 30 minutes ....
ActivityMonitor(L" ");
ActivityMonitor(szEndMsg);
fwprintf(hLog, szEndMsg);
InvalidateRect( hWnd, &rcMonitor, TRUE ); // invalidate the monitor area
UpdateWindow(hWnd);
//RedrawWindow(hWnd, &rcMonitor, 0, RDW_UPDATENOW);
fclose(hLog);
DisplaySettings();
szInpPath[nIP_End] = L'\0'; // restore input path
szOutpPath[nOP_End] = L'\0'; // restore output path
}
break;
}
case IDC_F_BACKUP:
nIP_End = SelectPath(hWnd, szInpPath, L"Select the folder to be backed up");
DisplaySettings();
break;
case IDC_F_SAVEAS:
nOP_End = SelectPath(hWnd, szOutpPath, L"Create a folder for this backup");
DisplaySettings();
break;
case IDC_VERSION:
MessageBoxW( NULL, L"\n FolderClone.exe Version 1.1 /MN"
L"\n Create a zip-backup of the selected"
L"\n directory (archive all files per folder,"
L"\n directory structure left unchanged).", L" ? ",MB_OK );
break;
case IDC_F_EXIT:
PostMessage(hWnd, WM_CLOSE, 0, 0);
break;
}//switch(LOWORD(wParam))
break;
default:
return DefWindowProcW(hWnd, msg, wParam, lParam);
}//WM_COMMAND
return 0L;
}//switch(msg)
return (0L);
}