Ubuntu – Ubuntu 16.04 update manager error

16.04ubuntu-gnomeupdate-manager

Every time I attempt to perform a software update I get this error from the software manager:

W:GPG error: http://download.virtualbox.org/virtualbox/debian xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A2F683C52980AECF, W:The repository 'http://download.virtualbox.org/virtualbox/debian xenial InRelease' is not signed., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., W:There is no public key available for the following key IDs:
A2F683C52980AECF  , W:The repository 'http://ppa.launchpad.net/blackmage/f.lux/ubuntu xenial Release' does not have a Release file., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., W:The repository 'http://ppa.launchpad.net/gezakovacs/ppa/ubuntu xenial Release' does not have a Release file., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., W:The repository 'http://ppa.launchpad.net/zeal-developers/ppa/ubuntu xenial Release' does not have a Release file., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., W:http://deb.playonlinux.com/dists/trusty/InRelease: Signature by key 74F7358425EEB6176094C884E0F72778C4676186 uses weak digest algorithm (SHA1), W:http://liveusb.info/multisystem/depot/dists/all/Release.gpg: Signature by key 32027DE3D67157C45E69C0AE4E940D7FDD7FB8CC uses weak digest algorithm (SHA1), W:http://vagrant-deb.linestarve.com/dists/any/InRelease: Signature by key AD319E0F7CFFA38B4D9F6E55CE3F3DE92099F7A4 uses weak digest algorithm (SHA1), E:Failed to fetch http://ppa.launchpad.net/blackmage/f.lux/ubuntu/dists/xenial/main/source/Sources  404  Not Found, E:Failed to fetch http://ppa.launchpad.net/blackmage/f.lux/ubuntu/dists/xenial/main/binary-i386/Packages  404  Not Found, E:Failed to fetch http://ppa.launchpad.net/gezakovacs/ppa/ubuntu/dists/xenial/main/binary-i386/Packages  404  Not Found, E:Failed to fetch http://ppa.launchpad.net/zeal-developers/ppa/ubuntu/dists/xenial/main/source/Sources  404  Not Found, E:Failed to fetch http://ppa.launchpad.net/zeal-developers/ppa/ubuntu/dists/xenial/main/binary-i386/Packages  404  Not Found, E:Some index files failed to download. They have been ignored, or old ones used instead.

Here are some screenshots of what is displayed when I attempt to edit the repositories I have:

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I am not certain how to fix this.

Best Answer

You obviously did a system upgrade without having removed the external repositories before. The easiest way to solve the problem would be to remove all external repositories and delete all related public keys. After having done this, update the repositories (sudo apt-get update) and check whether everything is fixed now ... and when no error occurs anymore, you can add new external repositories, but carefully check whether these repositories are "matching Xenial" ...

Depending on which repositories you add, a "weak signature" warning still may appear when updating the repositories - but you can ignore it, because it is just an informational message.

gpg: Signature by key ... uses weak digest algorithm (SHA1)