Everything was working but suddenly stopped. I just didn't upgrade my Ubuntu 14.04 for two months. Now when I want to upgrade I get some errors.
First I run
sudo apt-get update:
E: GPG error: http://dl.google.com stable InRelease: Clearsigned file isn't valid, got 'NODATA' (does the network require authentication?)
I search for a solution and found this, but now when I try to import the key, I get another error. While importing the key:
wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo apt-key add -
I get this error:
gpg: no valid OpenPGP data found.
How to solve this problem?