Fixed it.
Edited imagebrowser.php and wrapped an excluding is_feed()-condition around the section being started with:
<div class="ngg-imagebrowser-nav">
<?php if (!is_feed()) { ?>
<div class="ngg-imagebrowser-nav">
<div class="back">
<a href="<?php echo $image->previous_image_link ?>">&#96
68; <?php _e('Back', 'nggallery') ?></a>
</div>
<div class="next">
<a href="<?php echo $image->next_image_link ?>"><?php _e
('Next', 'nggallery') ?> ►</a>
</div>
<div class="counter"><?php _e('Picture', 'nggallery') ?> <?php e
cho $image->number ?> <?php _e('of', 'nggallery')?> <?php echo $image->total ?><
/div>
<div class="ngg-imagebrowser-desc"><p><?php echo $image->descrip
tion ?></p></div>
</div>
<?php } ?>