Download plugin error code key() parameter 1 is null

Hi,

I get the following error code when guests get to a download page:

Warning: key() expects parameter 1 to be array, null given in /var/home/croize_at/www/interne2/main/wp-content/plugins/nmedia-user-file-uploader/inc/helpers.php on line 635

the plugin works fine though. Is there a fix or am I doing something wrong? File Manager and Download Manager Addon are updated (just installed 2 days ago)

Thanks