@jpcw

Then going from 10.1.4 to 10.2.0 I had to modify my file "/modules/contrib/ckeditor5_font/config/schema/ckeditor5_font .schema.yml" to use the schema defined in "/core/modules/ckeditor5 /config/schema/ckeditor5.schema.yml/"

So go from:

# Plugin \Drupal\ckeditor5\Plugin\CKEditor5Plugin\ListPlugin
ckeditor5.plugin.ckeditor5_list:
  type: mapping
  label:List
  mapping:
    reversed:
      type: boolean
      label: 'Allow reverse list'
      constraints:
        NotNull: []
    startIndex:
      type: boolean
      label: 'Allow start index'
      constraints:
        NotNull: []

has

# Plugin \Drupal\ckeditor5\Plugin\CKEditor5Plugin\ListPlugin
ckeditor5.plugin.ckeditor5_list:
  type: mapping
  label:List
  mapping:
    properties:
      type: mapping
      mapping:
        label: 'Allowed list attributes'
        reversed:
          type: boolean
          label: 'Allow reverse list'
          constraints:
            NotNull: []
        startIndex:
          type: boolean
          label: 'Allow start index'
          constraints:
            NotNull: []
    multiBlock:
      type: boolean
      label: 'Allow blocks to be created in list items'
      constraints:
        NotNull: []

in ""/modules/contrib/ckeditor5_font/config/schema/ckeditor5_font.schema.yml"

Don't forget to rename "/modules/contrib/ckeditor5_font/config/schema/ckeditor5.schema.yml" to "/modules/contrib/ckeditor5_font/config/schema/ckeditor5_font .schema.yml" if this is not already done

same problem going from 10.1.1 to 10.2
for my part it came from the module "ckeditor5_font"
I modified the name of the file "/modules/contrib/ckeditor5_font/config/schema/ckeditor5.schema.yml" to "/modules/contrib/ckeditor5_font/config/schema/ckeditor5_font .schema.yml"
and everything is back to normal