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 = 12M to your php.ini file (recommended, if you have access)
• ini_set('memory_limit', '12M'); in your sites/default/settings.php file
• php_value memory_limit 12M in 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.