The usual cause of this is server resource exhaustion and often happens in a shared hosting environment. Other factors could also cause this. Please check for the following.
Ensure everything is up to date.
If you're using a caching plugin, clear all caches including browser cache then deactivate your caching plugins and other optimization plugins.
If you're using a CDN, please clear the CDN's cache and disable optimization services.
Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Reset your htaccess file by renaming it to
.htaccess-bak. Then in WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.
Contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
If nothing helps, please give us WP admin and cPanel access in a Secure Note.