That's because you haven't added the GPG key for Medibuntu. Please do:

sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo
apt-get update

Please refer to
https://help.ubuntu.com/community/Medibuntu#Adding%20the%20Repositories

** Changed in: update-manager (Ubuntu)
       Status: New => Invalid

-- 
Malformed Release file when updating to 8.04 with update manager
https://bugs.launchpad.net/bugs/213890
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