What is this Fatal error?

[Lynne]

That error indicates that your php doesn't have enough memory allocated to complete the operation. You can either increase your memory_limit setting in php.ini to xxM (something larger than what you currently have) or to temporarily up your limits edit your includes/config.php file and add this line right under the <?php line:

ini_set("memory_limit", "xxM");

(You can view your current memory_limit in Maintenance > View PHP Info . If it's 16M, try 32M. If it's 32M, try 64M. If it's 64M, try 92M or 128M, etc.)

But, if you are only having the issue when rebuilding, then try a smaller number of posts as suggested.