Okay...hab den Fehler gefunden....mein Modulordner heißt nicht "Gallery", sondern "Coppermine".
Ich habe entsprechend in der codebase.php die Zeilen 31-41 angepasst:
// Function to modify the gallery header
function LightBoxJS_header($html) {
$html = '<link rel="stylesheet" href="modules/Coppermine/plugins/light_box/slideshow/css/lightbox.css" type="text/css" media="screen" />'
.$html = '<script type="text/javascript" src="modules/Coppermine/plugins/light_box/slideshow/js/prototype.js"></script>'
.$html = '<script type="text/javascript" src="modules/Coppermine/plugins/light_box/slideshow/js/scriptaculous.js?load=effects"></script>'
.$html = '<script type="text/javascript" src="modules/Coppermine/plugins/light_box/slideshow/js/lightbox_s.js"></script>'
...und jetzt läuft es!

Gruß
Bernd