thx ivan, your solution works.
darkvipers patch is maybe safer for future.
@darkviper:
i think my changes was wrong in the save.php. but now it should be ok. is this right so?
<?php
// Update the mod_wysiwygs table with the contents
$MEDIA_URL = WB_URL.MEDIA_DIRECTORY;
if(isset($_POST['content'.$section_id])) {
$content = $_POST['content'.$section_id];
$searchfor = '#(<.*= *\")('.quotemeta($MEDIA_URL).')(.*\".*>)#iU';
$content = preg_replace($searchfor, '$1{SYSVAR:MEDIA_REL}$3', $content);
$content = $admin->add_slashes($content);
// searching in $text will be much easier this way
$text = umlauts_to_entities(strip_tags($content), strtoupper(DEFAULT_CHARSET), 0);
$query = "UPDATE ".TABLE_PREFIX."mod_wysiwyg SET content = '$content', text = '$text' WHERE section_id = '$section_id'";
$database->query($query);
}