This error happens when your memory is not big enough to execute the php code (uploading large images, deleting numerous products, sending mass mails etc). You would then have to enlarge the memory allocated for PHP.
Fatal error: Allowed memory size of 536870912 bytes exhausted
Apply one of solutions bellow to increase the limit to 64MB, 128MB, 256MB or 512MB, depending on host.
1. Edit php.ini
memory_limit = 128M;
2. Or put code below to .htaccess
php_value memory_limit 128M
If unluckily neither solution works, contact your hosting provider For most shared hosting, there is maximum memory_limit. You can’t set memory-limit to 64Mb if you get max 32Mb.