Comment 34 for bug 409621

As pointed out a few comments ago, the actual code in libxklavier where this error occurs is this:

#ifdef HAVE_XINPUT
 if (xkl_engine_priv(engine, features) | XKLF_DEVICE_DISCOVERY) {
  DevicePresence(display, xitype, xiclass);
  XSelectExtensionEvent(display,
          xkl_engine_priv(engine, root_window),
          &xiclass, 1);

There is an error with that function in it's current form (the if statement always evaluates true, even if the X server has no XIE). However, that isn't your issue here. The log from g-s-d clearly shows that your X server has XIE loaded:

[1250597338,150,xklavier_xkb.c:xkl_xkb_init/] XInputExtension found (131, 66, 129)

...and your output of xdpyinfo supports this too. With that in mind, I can't think of any reason why this code section would cause this error, other than an issue with NX itself. I've spoken with the maintainer of libxklavier too, and we agreed that this should be assigned to FreeNX now (at least until we see a similar issue with Xorg)