500 on frontend but page still shows

It’s weird as the page still is being rendered properly even with the error.
Here is the error from the logs:

PHP Warning: DOMDocument::createTextNode() expects parameter 1 to be string, object given in /www/wp-content/plugins/psn-pagespeed-ninja/ress/classes/htmloptimizer/dom.php on line 1088 when calling DOMDocument::createTextNode(), request: GET /

Any ideas why or how to fix it?