You have such problem, because Your PHP configuration at the server has a memory limit, which is rather low.
I'd recommend to set it to 32 Mb.
!!!Please, contact your host provider to increase PHP's memory limit to 32 Mb.!!!
or if you have access to php.ini follow in this way:
If so, search for memory_limit and change the value to 32M
If not, do you have a .htaccess file in your Joomla website root folder?
If so, add the following line:
php_value memory_limit 32M