Symptoms: A task is scheduled in Plesk under Scheduled Tasks, the task attempts to run a php script but fails.

Solution:
By Default Plesk 11 does not allow execution of scripts in a non-chrooted environment,

To Allow execution of Scripts, change the server policy from the command prompt:

# /usr/local/psa/bin/server_pref -u -crontab-secure-shell “/bin/sh”

To disallow:
# /usr/local/psa/bin/server_pref -u -crontab-secure-shell “”