Same issue here...
Doing a fresh installation of Plesk 12.x on Ubuntu 12.04 LTS -- getting the same error: -
"The package https://autoinstall.plesk.com/PSA_12.0.18/microupdates/MU5/common/_usr_local_psa_admin_bin_plesk_agent_manager could not be downloaded because it could not be found (The...