Ubuntu – Update from 14.04 to 16.04 failed with GPG error

14.0416.04gnupgupdates

apt-get update after upgrade – I get following message

W: GPG error: http://archive.canonical.com/ubuntu xenial InRelease: At least one invalid signature was encountered.
W: The repository 'http://archive.canonical.com/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ua.archive.ubuntu.com/ubuntu xenial InRelease: At least one invalid signature was encountered.
W: The repository 'http://ua.archive.ubuntu.com/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ua.archive.ubuntu.com/ubuntu xenial-security InRelease: At least one invalid signature was encountered.
W: The repository 'http://ua.archive.ubuntu.com/ubuntu xenial-security InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ua.archive.ubuntu.com/ubuntu xenial-updates InRelease: At least one invalid signature was encountered.
W: The repository 'http://ua.archive.ubuntu.com/ubuntu xenial-updates InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ua.archive.ubuntu.com/ubuntu xenial-proposed InRelease: At least one invalid signature was encountered.
W: The repository 'http://ua.archive.ubuntu.com/ubuntu xenial-proposed InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ua.archive.ubuntu.com/ubuntu xenial-backports InRelease: At least one invalid signature was encountered.
W: The repository 'http://ua.archive.ubuntu.com/ubuntu xenial-backports InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Tried

sudo apt-get clean
sudo mv /var/lib/apt/lists /tmp
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get clean
sudo apt-get update

It did not help.

My current source.list:

###### Ubuntu Main Repos
deb http://ua.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse 

###### Ubuntu Update Repos
deb http://ua.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse 
deb http://ua.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse 
deb http://ua.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse 
deb http://ua.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse 

###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu xenial partner

Best Answer

Fixed it with rm -rf /etc/apt/trusted.gpg.d/*

Apparently, gpg of some old repository was messing things up, despite those repositories being disabled..

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