Why does libvirtd fail to authenticate user via virt-manager as root with 'authentication failed' error ?

Issue

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/connection.py", line 1154, in _open_thread
    self.vmm = self._try_open()
  File "/usr/share/virt-manager/virtManager/connection.py", line 1138, in _try_open
    flags)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 102, in openAuth
    if ret is None:raise libvirtError('virConnectOpenAuth() failed')
libvirtError: authentication failed: authentication failed