This is the default behaviour of Automatix, it has no sweet way to fail. It has nothing to do with update manager. When you installed automatix it broke your repos. It took the good /etc/apt/sources.list and overwrote them.
** Changed in: update-manager (Ubuntu Dapper)
Status: Unconfirmed => Rejected
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Repositories options problem
https://launchpad.net/bugs/50123
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