I might have found the source of the issue, I'm not sure, because I cannot make modifications via FTP. To try my theory, please change the following:
1. Open the wp-content\plugins\related-posts-pro\includes\related_content.class.php file and navigate to line 1492, which should be this:
add_shortcode('wpdreams_ajaxsearchpro', array($this, 'return_empty_string'));
2. Remove that line, or comment it like this:
//add_shortcode('wpdreams_ajaxsearchpro', array($this, 'return_empty_string'));
3. Save the file, and go to the back-end and save the Related Posts Pro options as well.
Let me know if it helps, so I can investigate it further on my test environments.Best,
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