Multibyte Support enabled
I did some more testing with the "scandir" function in PHP and return this array to see what happens and it display the content correctly. So I presume there is something wrong with the fileshare module in Drupal.
Because this is not related to ISPConfig nor the Ubuntu howto here I stop bothering you here
I already submitted this problem in the fileshare bug tracker and wait for an answer of the developer.
Thanks a lot for your support!