Thank you for your advice, Juanito.
I think I use Xvesa. Because I have never done anything to these servers yet.
I did the following:
$ cat .xsession|grep Xvesa (This gave me some messages.)
$ cat .xsession|grep Xfbdev (Nothing happened.)
$ cat .xsession|grep Xorg-7.7 (Nothing happened.)
$ cat .xsession
(Xvesa -br -screen 1024x768x32 -shadow -2button -mouse
/dev/input/mice,5 -nolisten tcp -I >/dev/null 2>&1 &
export XPID=$!
waitforX || ! echo failed in waitforX || exit
"$DESKTOP" 2>/tmp/wm-errors &
export WM-PID=$!
[ -x $HOME/.setbackground ] && $HOME/.setbackground
[ -x $HOME/.mouse_config ] && $HOME/.mouse_config &
[ $(which "$ICONS".sh) ] && ${ICONS}.sh &
[ -d "$HOME/.X.d" ] && find "$HOME/.X.d" -type f -print | while read F;
do. "$F"; done)