My custom title gets duplicated at every save or post update

First of: Your custom title suffix getting duplicated might not be what you want to happen, but is expected behavior as per your code sample. The save_post action hook runs everytime a post (or page) is created or updated. Hence your callback runs everytime. Let me offer two solutions: 1 Use the wp_insert_post action instead …

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Connection Reset on post/page save

If you are running mod_security or anything like that would be first thing to check. Security modules are server are known to react badly to WP’s POST requests. That is consistent with circumstances you describe (post saves, Ajax).