Fatal Error While Importing

Hi,

I am seeing this as a fatal error when importing users.

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 12288 bytes) in /var/www/vhosts/mymotherscousin.com.au/httpdocs/wp-includes/functions.php on line 651

The code under functions.php is as below, but I cant see where the issue is and how to fix it:

/**
 * Unserialize data only if it was serialized.
 *
 * @since 2.0.0
 *
 * @param string $data Data that might be unserialized.
 * @return mixed Unserialized data can be any type.
 */
function maybe_unserialize( $data ) {
	if ( is_serialized( $data ) ) { // Don't attempt to unserialize data that wasn't serialized going in.
		return @unserialize( trim( $data ) );
	}

	return $data;
}

Please help.

Rami