Please post your lts.conf file. It should be in /opt/ltsp/i386/etc on your server if you've installed to the default directory. If not, type 'find / -name lts.conf' and it'll tell you.
While you're waiting for a reply, try altering your lts.conf file thusly:
Change the XSERVER = <something> to XSERVER=vga or XSERVER=svga.
Add the line X_MODE_0 = 640x480
Add the line X_COLOR_DEPTH = 8
More on the lts.conf file can be found here:
http://www.ltsp.org/documentation/lt....0.html#AEN903
The docs are in different languages as well. Spanish is there, but not Portuguese.
I ran into the same problem as you in setting up the Xserver. I was trying to use very wierd video cards. I finally stuck in a Cirrus Logic card and everything fixed itself.