BUSY: Update operation was locked by another update process.
Having trouble updating Plesk? Check for the installer:

ps aux | grep installer

root     31859  0.0  0.2 135560 20820 ?        S    21:10   0:00 /var/cache/parallels_installer/parallels_installer_CentOS_6_x86_64 --service-mode --enable-xml-output --ssl-cert /usr/local/psa/admin/conf/httpsd.pem --branch release,stable --web-interface --with-ssl --disable-browser

Kill the process

kill -9 31859

There’s a psa-installer.lock file in /tmp/ which indicates Plesk a running installer. Be sure no installer is running and then delete this little bugger.

# cd /tmp
# ls
autoinstaller3.log  autoinstaller_webui_socket  phpsqGbDE  phpxQn6nH  psa-installer.lock  spamd_full.sock