Mapproxy tiles are partly black instead of transparent

Question

I use MapProxy for caching WMS/WMTS tiles and it works fine up to a certain zoom level. But for higher zoom levels MapProxy fills the empty parts of the layers in black. This happens for certain geometries, mostly polygons where I only showe their stroke but they have an empty fill.

Here a comparison of the tiles requested to QGIS server and through MapProxy. First the tile requested directly from QGIS server:

And here the tile requested through MapProxy:

It seems to be a problem with transparencies as the white parts also should be transparent. I define transparent true at the sources configuration in mapproxy.yaml:

sources:
  test_wms:
    type: wms
    req:
      url: http://qgis-server.com/service?
      layers: Test WMS
      transparent: true

I additionally tried (without success) to define a global format and use in in cache:

globals:
  image:
    formats:
      imgformat:
        format: image/png
        mode: P
        transparent: true

Another test was to add the image parameter to the layer source configuration:

image:
  transparent_color: '#ffffff'
  transparent_color_tolerance: 0

Answer 1

Finally solved the issue following the advice of a message on MapProxy mailing list. Seems to be a problem with with an old version of Pillow on Ubuntu 16.04.

Update to latest version (and than reboot!) did the trick:

pip install Pillow==5.1.0