How to revert software sources back to default? [duplicate]

I don't think it has anything to do with the add-on but I had a similar problem. Somehow your sources.list is modified on line 2. Do this:

Make a copy of your sources.list, just in case you need to revert the changes:

sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup

Open the file with an editor, like:

gksu gedit /etc/apt/sources.list

In the second line you'll find the problem: That sentence:

See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

has to be commented, otherwise the update manager will try to read it as a source and that's the error you are seing.

Simply add a # in front of that line. I should appear like this:

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

If the problem persist, maybe because you have other lines with the same error, just correct'em all. But DON'T comment all the lines, first try to find which ones are wrong. Run update-manager each time and if there's an error it will tell you the line where it is.

Good luck!