Looks like X doesn't know what that mode is. Or the XFree86 developers figured "almost nobody can even use this mode because monitors don't get that big, so let's not support it" (this would have been a while ago; X 4.2 isn't all that new). But I kinda doubt that last one.
Or your monitor is telling X that that mode isn't supported, however I don't think this is your problem either. The error message sounds like X doesn't recognize the mode as being even a candidate for being valid. If you had told it to go into 1843x1567 mode or some gibberish like that, I have a feeling it would complain the same way.
I have no idea how to fix it, either, short of looking at the XFree86 source and adding support for that display mode. And I have no idea how you'd go about doing that; I don't know how it works internally that well.