Your sources.list looks fine. You can't run /etc/apt/sources.list as it's not a program/script (only a text file). Hence the error when using sudo. If those commands work, you've looked at the sources.list files with the first command and the second one downloads and installs all of the updates. It looks like your update manager program is somehow corrupted. First try running
to try to repair any damaged packages and then run Update Manager again.Code:sudo apt-get install -f
If you still get an error it might be worth trying a re-install via Synaptic Package Manager. Search for update-manager. The two things you want to reinstall are update-manager and update-manager-core. Right click on the green box next to them and select Mark for Reinstallation and then click on Apply. If this doesn't work I'm not sure what to suggest.