admin_functions.php:
Find:

<select id="album_watermark" name="album_watermark">

Change to:

<select id="<?php echo $javaprefix; ?>album_watermark" name="<?php echo $prefix; ?>album_watermark">

Find:

$new = sanitize($_POST['album_watermark'], 3);

Change to:

$new = sanitize($_POST[$prefix.'album_watermark'], 3);

Note that the mass edit plugin is no longer part of Zenphoto and is no longer needed nor supported.