Tirex / Mapnik does not find font

I am getting the following Tirex error:

tirex-backend-mapnik[15024]: no valid fonts could be loaded in FontSet 'fontset-0' in FontSet at line 17 of '/etc/mapnik-osm-carto-data/osm.xml'

The first font in osm.xml is DejaVu Sans Book which is installed:

# python -c "from mapnik import FontEngine as e;print '\n'.join(e.instance().face_names())" | grep "DejaVu Sans Book"

Output:
DejaVu Sans Book

When rendering via generate_image.py it works.