I changed to a new Synology NAS from DSM 5.2 to 6.1. After migrating my WordPress site Permalinks worked only in default mode. On my old NAS i used to have something like “/2013/10/file-open-security-warning/” .
The permalink update is not running after a change. So all posts ended up in 404 Errors.
Changing the website server from Nginx to Apache within virtual host settings solved the Problem.