I would recommend you to disable all of PageSpeed Ninja optimization, and then enable them one-by-one and re-check scores in Google Page Insights to found which optimization is related to 500 error. Note that Google Page Insights caches results for “few minutes”, so it is necessary to either re-check with few minutes interval, or add a dummy query to URL, e.g. https://rakhioverseas.com/?test1 .
And what web server do you use? HTTP headers say it Apache, but the header may be set arbitrary. If it is Apache, which version, 2.2 or 2.4? And how PHP is called, as Apache module or via FastCGI?
Additionally, you could try to enable Error logging in Troubleshooting section of Advanced tab of PageSpeed Ninja settings, it will store all errors, warnings, and notices to wp-content/plugins/psn-pagespeed-ninja/includes/error_log.php, maybe it will be possible to found something related there.