How do I track down x_create_frame "Unable to create window" on w32?

[Lennart Borgman]

I got a backtrace

Debugger entered--Lisp error: (error "Unable to create window")
  x-create-frame(((visibility) (icon-name) (top . 0) (left . 0)
(unsplittable) (modeline . t) (width . 60) (height . 20)
(background-mode . dark) (alpha 100 . 10) (scroll-bar-width . 0)
(cursor-type . box) (auto-lower) (auto-raise) (icon-type) (fullscreen)
(buffer-predicate) (tool-bar-lines . 0) (menu-bar-lines . 0)
(right-fringe . 0) (left-fringe . 0) (line-spacing) (screen-gamma)
(vertical-scroll-bars) (internal-border-width . 0) (border-width . 2)
(font-backend uniscribe gdi)))

and wondered how to track it down.

I believe the error comes from this in w32_window in w32fns.c:

  if (FRAME_W32_WINDOW (f) == 0)
    error ("Unable to create window");

I added some DebPrint to w32_createwindow to see what was wrong:

  if (!hwnd)
    {
      DebPrint (("w32_creatwindow CreateWindow => %d", GetLastError ()));
    }

but to my surprise I got no output in dbg from it.

I don't understand. Help!