Thank you very much for the usefull links and the tip. Yes, the vhost default was not default. Now I've set it to default explicitly with the flaf default_server and everything works fine!

Thank you, Valentin!

Since "default server is the first one" (http://nginx.org/en/docs/http/request_processing.html), the problem must have been, that the server couldn't find a vhost for the request and just took the first vhost. But what ist "the first" vhost, when all vhosts are stored in different files? In which order are the vhost files processed?