Tinnitus
Can you go to the download page on our website, click Language Packs, select v1.17.3, select the German language pack, download, upload this new pack to your server, overwrite the old one, and retest?
If you run into the same issue then you will need to review your logs (general server logs, webserver error logs, PHP error logs, MySQL/MariaDB error logs, osTicket System Logs, Browser Console logs, etc.) for any related errors.
Cheers.