I have problems using mapproxy with QGIS-server via openlayers (mapproxy-1.6.0, qgis-server 2 and openlayers-2.13.1).
I have a layer in which i defined the background of the polygon with transparency.
When I asked directly qgis-server via openlayers, I get the png wanted (png background is entirely transparent and the transparency of the background is what i need).
When I pass through maproxy, the image background is transparent but not the background of the polygons (the setting of the mapproxy may be ok because I get a transparent png image). What's weird is when I set the background of a polygon with less than 50% transparency in QGIS, through mapproxy the background of the polygon is completely opaque and when I put a transparency equal to or greater than 50%, the background of polygons is completely transparent.
Exemple directly from qgis-server :
Exemple via mapproxy :