On my server "broek", the Owncloud web interface wasn't working. As it turned out, each request to a PHP page returned a file for download instead of serving the PHP page.

Based on this thread I tried dpkg -l | grep libapache and saw libapache-mod installed, but httpd.conf was empty.

So I resorted to removing PHP altogether:

sudo apt-get remove php5

Unfortunately this uninstalled Owncloud too.

And reinstalled PHP:

sudo apt-get install php5
sudo apt-get install libapache2-mod-php5

After restarting apache server, serving PHP files worked again. So I tried reinstalling Owncloud:

sudo apt-get install owncloud

The Owncloud web interface then worked again and even remembered the configuration!