[MapProxy] Error on some zoom levels

[Максим Хитрин]

Dear Sir or Madam,

I've got a problem when I'm trying to zoom in MapProxy.
It works well for about 2 or 3 zoom levels but then I got an error "Could
not get any sources: no image returned from source WMS"
I'm trying to get data from a MapServer that is localed on another machine.
There is no limit for max/min scale in its capabilities.

Here is my configuration:
sources:
  test3857:
    type: wms
    wms_opts:
      version: 1.3.0
    req:
      url:
http://85.143.48.160:1024/cgi-bin/mapserv.exe?map=C:/Map%20Files/test3857/test3857.map
      layers: QGIS-MAP
      transparent: true
    supported_srs: ['EPSG:3857']
    coverage:
      bbox: [6807775, 7373595, 6875170, 7422370]
      bbox_srs: 'EPSG:3857'

grids:
  test3857_grid:
    base: GLOBAL_GEODETIC
    bbox: [6807775, 7373595, 6875170, 7422370]
    bbox_srs: 'EPSG:3857'

caches:
  test3857_cache:
    sources: [test3857]
    grids: [test3857_grid]

layers:
  - name: test3857
    title: test3857
    sources: [test3857_cache]

Why am I getting this error?
Thank you for your prompt help.
--

Yours faithfully,

Maxim Khitrin
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