If the suggestions listed above do not help and if you have more than 500 visitors per day on your website, it may be necessary to readjust your PHP configuration. The more visitors you have, the more resources are required to generate the reports. The cruicial factors are the memory and the maximum execution time of PHP scripts.
The value/configuration that should be increased first is memory_limit. Depending on your host, this can be done in a number of places with the most likely being php.ini or .htaccess depending on your hosting situation. Add for example:
memory_limit = 196M to your php.ini file (recommended, if you have access)
With root access, you can use the sed util in Linux/Unix based systems, in order to increase the memory for 196M. Don't forget to properly locate your php.ini file!
sed -i 's/memory_limit = .*/memory_limit = 256M/' /etc/php5/apache2/php.ini
php_value memory_limit 256M to your .htaccess file in the LiveZilla root
If you are using an IIS webserver, please increase the Request & Activity Timeout value to at least 600 seconds. This page explains how to get to the configuration form where this can be set up.