I stumbled while installing a second WordPress blog. Everything seems to be fine: DB, PHP (first blog works), apache.conf, .. Nevertheless I received empty HTML bodies when calling a php page.
So I started debugging and finally found the problem inside my php.ini. In the moment I saw that line I remembered that I changed it to secure my server a bit
open_basedir = /path-to-first-wordpress-dir/:/path-to-second-wordpress-dir/:/tmp/
After adding the path of my second WordPress installation everything was fine.