Fatal error: Allowed memory size of X bytes exhausted (tried to allocate Y bytes)...
That indicates that Drupal needed more memory than PHP was allowed to give it.
Increase PHP's memory limit, either by adding:
memory_limit = 12Mto your php.ini file (recommended, if you have access)ini_set('memory_limit', '12M');in your sites/default/settings.php filephp_value memory_limit 12Min your .htaccess file in the Drupal root
Some hosts allow a PHP.ini in the root of your site.
Depending on the amount of modules you have enabled and their 'impact' on the site you may need to increase the memory_limit even more (sometimes to 30 MB or more). Experiment with what memory value works for your needs.

Restart your server
You may have to restart your server for this to take effect.