Gday Achim,
I was just letting you know that sometimes it's not possible to get zip working on server configurations but I will do my best to help you out.
It's probably a permissions error as mentioned in some of the comments here: http://php.net/manual/en/ziparchive.close.php
The best way for me to diagnose the issue is take a look at the plugin on your server. If this is ok with you please send login details to support@wpb2d.com
Cheers,
Mikey