"Let's do a apt-get update && apt-get upgrade to install a lot of fancy security updates!".

Well, somehow in this process my nginx got updated from 1.6.2 to 1.8.0 (I'm using dotdeb packages. I don't know why they made such a big jump?). The result were blank pages for my PHP scripts, without any errors, even after activating all debug switches I found.

After googling for my favorite key words "nginx php-fpm blank page no errors" and trying some things I fixed everything with this line in my PHP location block in the nginx configuration:

fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;

Maybe the nginx update overwrote the default fcgi_params file that's included by default, or it's not nginx' fault after all and PHP-FPM changed its behaviour. Whatever. Its fixed now and I hope the next update will be smoother. Like the 2000 updates before.