Hi there,
Our Domain Mapping services will only work if the domain name's nameservers have been pointed towards our servers. Setting a CNAME will not work correctly for your root domain.
If you'd like, you can set up a subdomain for your site, such as blog.jochenhung.de and set up a CNAME for that which points to weimarstudies.wordpress.com (you would need to create this CNAME within your hosteurope control panel).
If you'd like to use a subdomain rather than the root domain, let me know and I will set it up for you on this end!