theme-start @ Debian Sarge

It means another window manager (wm) is already running, you can’t just start Fvwm from another wm, it has to be started on a bare X-Session, assuming you are using startx to start X, but this in your ~/.xinitrc (and comment all possible previous entries out):

exec fvwm-theme-starter

Now start X by typing “startx” and everything should work alright.