The issue is specific to Ubuntu 14.04.
Plesk installation or execution of
sudo apt-get update or
sudo apt-get dist-upgrade gives the following warning:
Reading package lists... Done W: GPG error: http://autoinstall.plesk.com trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 227C38D6AF741DEB
This is Plesk internal issue #PPPM-3136, which has been fixed in Plesk 12.0.
Apply the latest Plesk microupdates.