MapProxy conf file illustrating a potential issue (MapError)

Conf file

services:
 demo:
 tms:
 wmts:
 wms:
     srs: ['EPSG:4326', 'EPSG:900913', 'EPSG:3857']

layers:
 - name: florida
   title: florida_layer
   sources: [florida]
 - name: mixed_florida
   title: mixed_florida_layer
   sources: [mixed_florida]

caches:
    florida:
        sources: []
        grids: [GLOBAL_GEODETIC]
        format: image/png
        cache:
            type: file
            directory_layout: tms
            directory: '/data/result'
    mixed_kabul:
        sources: [florida]
        grids: [GLOBAL_GEODETIC]
        format: mixed
        image:
            encoding_options:
                jpeg_quality: 75
        request_format: image/png
        cache:
            type: mbtiles
            filename: mixed_florida.mbtiles


globals:
 cache:
  base_dir: './mapproxy_cache'
  lock_dir: './mapproxy_cache/locks'
 image:
  resampling_method: nearest