Error while updating ubuntu 12.04 using update manager

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http://askubuntu.com – I have Ubuntu 12.04 installed in a sony vaio VPCEH38FN. I have internet connection through a proxy server. The update manager has suddenly stopped working. It gives me the following error, when I try to update, and then doesnt do anything else. What exactly is this error, and what should I do. Could not initialize the package information An unresolvable problem occurred while initializing the package information. Please report this bug against the 'update-manager' package and include the following error message: 'E:Encountered a section with no Package: header, E:Problem with MergeList /va (HowTos)