Traceback (most recent call last):
  File "/usr/bin/update-manager", line 99, in <module>
    from DistUpgrade.DistUpgradeController import DistUpgradeController
  File "/usr/lib/python2.7/dist-packages/DistUpgrade/DistUpgradeController.py", line 45, in <module>
    from DistUpgradeFetcherCore import country_mirror
ValueError: bad marshal data (unknown type code)