What’s the warning on line 736?

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As you can see here (https://core.trac.wordpress.org/browser/tags/5.0.3/src/wp-includes/class-wp-query.php#L736), the 736 line of that file contains:

$qv['name'] = trim( $qv['name'] );

And $qv is defined as:

$this->query_vars = $this->fill_query_vars($this->query_vars);
$qv = &$this->query_vars;

So… That means, that you use WP_Query incorrectly somewhere in your code…

You should pass string value as name param, and it looks like somewhere in your site an array is passed as that param…