One sitemap returns http error 500

Hi,

I use your plugin on a website for a woocommerce webshop, and one of the sitemaps keeps returning a http error 500. The main sitemap and all the others are working fine.

main: http://www.minervaantikvarium.hu/sitemap.xml

error 500: http://www.minervaantikvarium.hu/sitemap-posttype-product.xml

the error log says:
[29-May-2017 07:58:26 UTC] PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 5230 bytes) in /home/mbahwd/public_html/wp-includes/functions.php on line 435
[29-May-2017 07:58:26 UTC] PHP Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0

The wp-includes/functions.php (line 435 strong):

/**
* Serialize data, if needed.
*
* @since 2.0.5
*
* @param string|array|object $data Data that might be serialized.
* @return mixed A scalar data
*/
function maybe_serialize( $data ) {
if ( is_array( $data ) || is_object( $data ) )
return serialize( $data );

// Double serialization is required for backward compatibility.
// See https://core.trac.ww.wp.xz.cn/ticket/12930
// Also the world will end. See WP 3.6.1.
if ( is_serialized( $data, false ) )
return serialize( $data );

return $data;
}

Wordpress and all plugins are up to date. We have around 1200 simple products in the webshop, so there should be 1200 urls, each with the featured image attached.

I’d really appreciate if you could help to solve this.
Thanks in advance.