Content Moderation: Editorial workflow not installed and broken/missing handler on the moderated content view
The guys over at Drupal's Slack channel pointed me in the right direction. I added detailed troubleshooting infos to the Content Moderation Guide.

If you enable Content Moderation (drush en content_moderation) on a site, that was not installed using Drupal's Standard Profile as a base, you will run into some problems which require a few manual steps before you can use Content Moderation, as detailed in the guide.

Fix missing Editorial workflow

If the Editorial workflow was not created when enabling Content Moderation, you can either manually create a new workflow and assign a content type, that's used with your new workflow or import the Editorial workflow manually.

The template for the Editorial Workflow is:

langcode: en
status: true
dependencies:
  module:
    - content_moderation
id: editorial
label: Editorial
type: content_moderation
type_settings:
  states:
    archived:
      label: Archived
      weight: 5
      published: false
      default_revision: true
    draft:
      label: Draft
      published: false
      default_revision: false
      weight: -5
    published:
      label: Published
      published: true
      default_revision: true
      weight: 0
  transitions:
    archive:
      label: Archive
      from:
        - published
      to: archived
      weight: 2
    archived_draft:
      label: 'Restore to Draft'
      from:
        - archived
      to: draft
      weight: 3
    archived_published:
      label: Restore
      from:
        - archived
      to: published
      weight: 4
    create_new_draft:
      label: 'Create New Draft'
      to: draft
      weight: 0
      from:
        - draft
        - published
    publish:
      label: Publish
      to: published
      weight: 1
      from:
        - draft
        - published
  entity_types:
    node: null
  default_moderation_state: draft


Copy the template to some yml file and import it using drush config:import --partial.

Fix Broken/missing handler on the Moderated Content view

This error occurs, if there exists no moderation workflow that is assigned to a content type. Wether you just imported the Editorial workflow as described above or created a custom moderation workflow, you'll have to make sure it has at least one content-type assigned to it. Just edit your moderation workflow, scroll to the bottom of the page and select at least one content type that is used with this workflow. After you save your changes, the "Broken/missing handler" on the Moderated Content will be gone and the view at /admin/content/moderated will work as expected.