Cluster Monitoring Operator degraded: "ConfigMap could not be parsed: error converting YAML to JSON"

Issue

The cluster monitoring operator is degraded with the error:

apiVersion: config.openshift.io/v1
kind: ClusterOperator
metadata:
  creationTimestamp: '2019-10-30T09:12:27Z'
  generation: 1
  name: monitoring
...
spec: {}
status:
  conditions:
    - lastTransitionTime: '2020-05-25T09:25:32Z'
      message: >-
        Rollout of the monitoring stack failed and is degraded. Please
        investigate the degraded status error.
      reason: InvalidConfiguration
      status: 'True'
      type: Upgradeable
    - lastTransitionTime: '2020-05-25T09:25:32Z'
      status: 'False'
      type: Available
    - lastTransitionTime: '2020-05-22T18:12:52Z'
      status: 'False'
      type: Progressing
    - lastTransitionTime: '2020-05-25T09:25:32Z'
      message: >-
        Failed to rollout the stack. Error: the Cluster Monitoring ConfigMap
        could not be parsed: error converting YAML to JSON: yaml: line 44: did
        not find expected key
      reason: InvalidConfiguration
      status: 'True'
      type: Degraded