[syslinux] Problems with gfxboot.c32

[Andreas Heinlein]

Hello,

I am currently trying to get the gfxboot archive from Ubuntu 12.04 
working with gfxboot.c32 under BIOS, using ISOLINUX 6.

AFAIK, the Ubuntu patches have been added in 4.03, and in fact it works 
with the official 4.05 release, but not with 5.10 or 6.01-pre5.

I can tell that the menu is actually working, you just don't see it. If 
you navigate it "blind", the correct entries start as usual.
What I get is a blank screen with a little gray rectangle in the bottom 
left corner, whereas a picture from "access.pcx" should be displayed 
instead. If you press "Esc" to get to the actual menu, the rectangle 
disappears and the screen is all blank. If you then press "Esc" again to 
leave gfxboot mode, the usual "You are about to leave GUI mode...." 
dialog appears, except that there is no text, just the window frame and 
the buttons. You can enter text mode from here, and then everything 
works like it should.

Any ideas how to debug this?

Thanks,
Andreas