[MapProxy] difference between screen and print

[Hälßig, Frank]

Dear list,

I have a strange behavior by using mapproxy and hope you can help me.
I use 2 layers (Flurstueck,Flurstuecksnummer) of a public WMS (https://geodienste.sachsen.de/wms_geosn_flurstuecke/guest?).
For testing purposes I used Q-GIS 2.8 (Ubuntu 16.04) and Q-GIS 3.0 (Win8.1). Mapproxy runs under Ubuntu 16.04.
At the screen everything works fine, but when I printout a map to pdf, the cadastral numbers are to small  (https://www.haelssig.net/temp/mapproxy/wur_mp.pdf)

Using the WMS direct in Q-GIS the printout to pdf is OK (https://www.haelssig.net/temp/mapproxy/wur_orig.pdf)

I tried MapProxy with cache  and direct source, the result ist the same.
When running MapProxy in develop mode I can see an extra parameter (&dpi=300) in the request for the print .
I have the same effect when using http://www.geoproxy.geoportal-th.de/geoproxy/services/INSPIREcp? .

How to start troubleshooting ?

Configuration is as followed:

grids:
  sachsen:
    base: GLOBAL_WEBMERCATOR
    bbox_srs: 'EPSG:25833'
    bbox: [269041,5551098,509220,5740594]
    srs: 'EPSG:25833'

layers:
  - name: Flurstuecke
    title: 'Flurstuecke'
    sources: [sn_fst_cache]

caches:
  sn_fst_cache:
    grids: [sachsen]
    sources: ['sn_gmk_wms:Flurstueck,Flurstuecksnummer']

sources:
  sn_gmk_wms:
    type: wms
    wms_opts:
      version: 1.3.0        
    req:
     url: https://geodienste.sachsen.de/wms_geosn_flurstuecke/guest? 
     layers: Flurstueck,Flurstuecksnummer,Gemarkung,Gemarkungsname
     supported_srs: 'EPSG:25833'
     coverage:
        srs: 'EPSG:25833'
        bbox: [276384, 5560720, 504054, 5728100]
    http:
      ssl_no_cert_checks: True
    image:
      transparent_color: '#FFFFFF'
      transparent_color_tolerance: 0

Best regards,
Frank Hälßig
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without disclaimer