Account has outdated last daily billing date, on attempt to bill an account manually, an exception occurs:
Account #ID billed with errors
In the logs an exception can be seen:
Daily Billing Process on Account #ID failed with exception: Table Resource Rate doesn't contain row with id Int32(NULL).
Account billing fails if on one of his subscriptions resources was deleted from the plan resource rates and the subscription was not synced with the changed plan. The issue is considered to be a bug with id #PBA-69771 ("Account Billing fails if resource rate has been deleted from the service plan").
Update to BA 6.0.7 as soon as it is available.
To fix an account billing before update installation either sync the subscription with the changed service plan or add the resource back to the service plan. The affected subscription ID, plan ID and resource ID can be found from the logs.