I can't update Ubuntu 10.04 LTS. The update manager says:
Could not download all repository indexes
The repository may no longer be available or could not be contacted because of network problems. If available an older version of the failed index will be used. Otherwise the repository will be ignored. Check your network connection and ensure the repository address in the preferences is correct.
GPG error: http://mirrors.softliste.de lucid/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9Failed to fetch http://ppa.launchpad.net/sevenmachin...64/Packages.gz 404 Not Found
Any ideas on how to fix this? Many thanks,