TaskDialogIndirect is returning an unusual error code

I'm using TaskDialogIndirect to display prompts to the user. Normally this works just fine, but sometimes, after the program has been running for a while, it begins returning an error code that the MSDN entry does not list as being one of the error codes this function can return.

0x80040001 OLE_E_ADVF "Invalid advise flags"

I have checked all the inputs to the function against previous successful calls in the same run. Aside from differences in the string to be displayed, they are identical. (the strings are even the same length.)

// create task dialog struct
TASKDIALOGCONFIG tdc;
ZeroMemory(&tdc, sizeof(TASKDIALOGCONFIG));
tdc.cbSize = sizeof(tdc);
tdc.dwFlags = (((dwMessageBoxFlags & MB_OKCANCEL) == MB_OKCANCEL) ? TDF_ALLOW_DIALOG_CANCELLATION : 0) | TDF_POSITION_RELATIVE_TO_WINDOW;
tdc.hwndParent = hwndOwner;
tdc.hInstance = LGetHInstance();
tdc.pszContent = usrText.wsz;
tdc.pButtons = _pButtons; 
tdc.cButtons = nButtons;
tdc.pszMainIcon = pszTaskDialogIcon;
tdc.pszWindowTitle = usrCaption.wsz;
tdc.nDefaultButton = nDefaultButton;
// display it now
int iButton = 0;
BOOL b = 0;
HRESULT hResult = TaskDialogIndirect(&tdc, &iButton, NULL, &b);

NEW INFORMATION

At the same time that TaskDialogIndirect stops behaving correctly, ShellExecute also stops working, as does CreateFile.

This was actually caused by an event handle leak elsewhere. When the available handles ran out, no more API calls which needed to create a handle could succeed. They did return a rather odd set of error codes though, none of which was "out of handles".