A common cause of the HTTP error in WordPress is a lack of memory. It can also result in many other issues such as the 503 service unavailable error. Fixing low memory is a matter of adding the following line to your wp-config.php file:
define (‘WP_MEMORY_LIMIT’, ‘256M’);
The above code increases your PHP memory limit to 256MB. Before adding it, you should also confirm what the maximum available memory limit is on your server by checking PHP info.
Also please check the issue by switching it to the default WordPress theme. If the issue persists then the issue is not related to the theme.
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