It took me a while to find out, why this backup script would not start. Finally, i found this error message: "sudo: sorry, you must have a tty to run sudo". The sudo call is required to execute "clustat" and find out, which machine is the current master.
sudo is configured to execute "clustat" without password and works fine on the command line - so what's wrong here?
The solution is easy: some distributions enable the setting "requiretty". This looks like:
in visudo. After commenting out this setting (you must be root for doing this), my cronjob started without any trouble.