That happens because Magento tries to allocate more memory than what is available on the server (32 MB). You can try to increase the value of PHP memory_limit in .htaccess or a custom php.ini file.
In your .htaccess, add this line:
php_value memory_limit 128M (it may already be there, but with a different value)
If that does not help, rename “php.ini.sample” to “php.ini” and override the memory limit like this:
memory_limit = 128M
If you have root access to your server, you may also increase the memory_limit in the system-wide php.ini file.
If you do not operate your own server and the above steps does not work, ask your host kindly if they can increase the memory limit.