Allowed memory size of 67108864 bytes exhausted
Or some other byte number can be there too. In this example the memory limit was 64M, which wasn't enough, so contact your server host to increase the memory_limit.
You can check your hosting information by creating a file in your server, let's call it info.php, and put only this in it:
<?php phpinfo(); ?>
Then look at this file in your browser: http://example.org/info.php