Take a look here (in German): http://www.forum-raspberrypi.de/Thread-fritzbox-fernzugang-und-nginx-web-server-%C3%BCber-gleiche-ip (I assume you're German-speaking, as you're using a Fritzbox).
Anyway, one (maybe _the_) esential issue in your configuration sample is that the argument of proxy_pass needs to be a numeric IP URL rather than a DNS URL (I don't know why).
Hope that helps,
Michael
Anyway, one (maybe _the_) esential issue in your configuration sample is that the argument of proxy_pass needs to be a numeric IP URL rather than a DNS URL (I don't know why).
Hope that helps,
Michael