I had previously installed another test box, so I decided to take a deeper look at the settings, write a puppet manifest and prepare this new box to become a production server. However, after browsing the webapp, all graphs were broken.
After googling a while, reading forums and bugs, I decided to open the $GRAPHITE_HOME/webapp/graphite/render/views.py, adding log.rendering(...) statements (not the most elegant solution, I know).
By following the program workflow I found out it was entering a block after checking if it should remotely render the image. This feature is turned on/off by REMOTE_RENDERING = True/False, in local_settings.py.
After setting this to False the problem was solved.