[MapProxy] opacity not working?

[DeDuikertjes]

List,

Unfortunately I still cannot get transparency to work.

I've created a WMS-service to debug.
I've created a minimal mapproxy config with only the things to get it 
going in it
I've created a viewer to see the result.

If you look at the demo-viewer:
http://178.63.74.185/hosting/demo/show_source.html?layernames=Bedrijventerrein&test=true&opacity=1&url=http://test.tiles.md-kwadraat.nl/mp/service

you see that the overlay is fully opaque. My configuration is attached 
below.

Please help me with tracking down this problem.
Is there anything wrong with my config?
How can I track this problem. I really don't know where to start.

Any help greatly appreciated.

Marco


caches:
   _Bedrijventerrein:
     grids: [applas_grid]
     link_single_color_images: true
     sources: [wms_Bedrijventerrein]
globals:
   cache:
     base_dir: /var/applas_user_data/prod/cache/
     lock_dir: /var/applas_user_data/prod/cache/locks/
     meta_buffer: 255
     meta_size: [3, 3]
     minimize_meta_requests: true
grids:
   applas_grid:
     bbox: [-285401.91999999998, 22598.080000000002, 595401.92000000004, 
903401.92000000004]
     res: [3440.6399999999999, 1720.3199999999999, 860.15999999999997, 
430.07999999999998,
       215.03999999999999, 107.52, 53.759999999999998, 
26.879999999999999, 13.44, 6.7199999999999998,
       3.3599999999999999, 1.6799999999999999, 0.83999999999999997, 
0.41999999999999998,
       0.20999999999999999]
     srs: EPSG:28992
     tile_size: [256, 256]
layers:
- name: Bedrijventerrein
   sources: [_Bedrijventerrein]
   title: Bedrijventerrein
services:
   tms: null
   wms:
     image_formats: ['image/png','png']
     md:
       abstract: test for transparency.
       access_constraints: none
       contact: {address: none, city: none, country: The Netherlands, 
email: none,
         organization: none, person: none, phone: none,
         position: none, postcode: none}
       fees: None
       online_resource: http://tiles.md-kwadraat.nl/
       title: test for transparency.
     srs: ['EPSG:28992']
sources:
   wms_Bedrijventerrein:
     image: {opacity: 0.4}
     req: { layers: Bedrijventerrein, transparent: true, url: 
'http://178.63.74.185/fcgi-bin/mapserv?map=/var/www/html/shared/services/wmsproxy/wmsproxy.map'}
     supported_srs: ['EPSG:28992']
     type: wms

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