The follwing article describes the steps that can be taken to debug the ISPConfig 3 server scripts.
Enable the debug Loglevel in ISPConfig
Login to the ISPConfig intterface and set the log level to Debug under System > System > Server Config (see also chapter 18.104.22.168 of the ISPConfig 3 manual) for the affected server. After one or two minutes, there should be more detailed messages in ISPConfig's system log (Monitor > System State (All Servers) > Show System-Log).
Disable the server.sh cronjob
Go to the command line of the server on which the error happens (on multiserver systems, it is often the slave and not the master) and run (as root):
Comment out the server.sh cron job:
#* * * * * /usr/local/ispconfig/server/server.sh > /dev/null >> /var/log/ispconfig/cron.log
Run the server script manually to get detailed debug output
Then run the command:
This will display any errors directly on the command line which should help you to fix the error. When you have fixed the error, please don't forget to uncomment the server.sh cron job again.