Why does leapp preupgrade fails with ModelViolationError: The value of "use_privilege_separation" field must be one of "sandbox, yes, no"

Issue

Leapp preupgrade fails with error

====> * read_openssh_config
        Collect information about the OpenSSH configuration.
Process Process-258:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 72, in _do_run
    actor_instance.run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 289, in run
    self.process(*args)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/opensshconfigscanner/actor.py", line 28, in process
    readopensshconfig.scan_sshd(self.produce)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/opensshconfigscanner/libraries/readopensshconfig.py", line 107, in scan_sshd
    produce_config(producer, config)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/opensshconfigscanner/libraries/readopensshconfig.py", line 82, in produce_config
    producer(config)
  File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 313, in produce
    self._messaging.produce(model, self)
  File "/usr/lib/python2.7/site-packages/leapp/messaging/__init__.py", line 204, in produce
    return self._do_produce(model, actor, self._new_data)
  File "/usr/lib/python2.7/site-packages/leapp/messaging/__init__.py", line 222, in _do_produce
    data = json.dumps(model.dump(), sort_keys=True)
  File "/usr/lib/python2.7/site-packages/leapp/models/__init__.py", line 124, in dump
    type(self).fields[field].to_builtin(self, field, result)
  File "/usr/lib/python2.7/site-packages/leapp/models/fields/__init__.py", line 146, in to_builtin
    target[name] = self._convert_from_model(getattr(source, name, None), name=name)
  File "/usr/lib/python2.7/site-packages/leapp/models/fields/__init__.py", line 93, in _convert_from_model
    self._validate_model_value(value=value, name=name)
  File "/usr/lib/python2.7/site-packages/leapp/models/fields/__init__.py", line 332, in _validate_model_value
    self._validate_choices(value, name)
  File "/usr/lib/python2.7/site-packages/leapp/models/fields/__init__.py", line 342, in _validate_choices
    'The value of "{name}" field must be one of "{values}"'.format(name=name, values=values))
ModelViolationError: The value of "use_privilege_separation" field must be one of "sandbox, yes, no"


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Actor read_openssh_config unexpectedly terminated with exit code: 1 - Please check the above details
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