I found the solution. The problem occurs, if OST gets emails through pipe.php. In this case two different system users have to share a resource, namely the folder, where the attachments are saved. The following steps helped me out:1) putting www-data (= webserver-user) and osticket (= php-cli-user) into a new group called "ticketattachments"2) creating an attachment-folder and changing it's group to "ticketattachments"1) chmodding the attachment folder to 2770, which sets it AND all files newly created inside to readwritable for the group3) Altering line 90 in plugins/strorage-fs/storage.php from "mkdir($base, 751)" to "mkdir($base, 770)"4) adding "umask 0002" to /etc/apache2/envvars5) adding "umask(0002);" to osticket/api/pipe.php right before "require_once(INCLUDE_DIR.'api.tickets.php');"Maybe some of this stuff is redundant, but it works. I hope, this will help someone.