It’s a known bug in Spip 1.6, with some versions of PHP (unless it’s a PHP bug ;)

To correct it manually, you have to modify 2 configuration files :

• in the file ecrire/inc_surligne.php3, look for the following line
  ob_start("");
  and replace it by ob_start();

This line is there four times. Change all four of them.

• You also have to modify (in the site root) the file inc.public.global.php3 replacing
  ob_start("");
  by ob_start()