Cannot add new items to multiple region

My client suddenly wasn't able to add new items to a multiple region anymore. This is my diagnostics report:

Perch: 2.8.29, PHP: 5.5.9-1ubuntu4.17, MySQL: 5.5.49, with PDO
Server OS: Linux, apache2handler
Installed apps: content (2.8.29), assets (2.8.29), categories (2.8.29), perch_blog (5.0)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', );
PERCH_LOGINPATH: /perch
PERCH_PATH: /var/www/eswirdbesser.at/perch
PERCH_CORE: /var/www/eswirdbesser.at/perch/core
PERCH_RESFILEPATH: /var/www/eswirdbesser.at/perch/resources
Image manipulation: GD Imagick
PHP limits: Max upload 20M, Max POST 8M, Memory: 128M, Total max file upload: 8M
F1: 2edba60ed1f613d6dd804feb202456a2
Resource folder writeable: Yes
HTTP_HOST: eswirdbesser.at
DOCUMENT_ROOT: /var/www/eswirdbesser.at
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php

There are already 110 Items in the region and it suddenly just didn't work anymore. There is no error message - the moment you click on "save", the page gets reloaded and all the contents, besides the thumbnails are filled out in the inputs, but it is not appearing as a new item or on the page. If you reload the page or just add another item, it disappears again.

Can you help me with this?