[Plugin: Related Posts for WordPress] SQL error when invoking function manually (only)

Hi,
I really like the plugin, good job.

I am having an issue when I try to call directly the API.
The interesting thing is that this error only occur when I call the API function, whereas the automatic hook seems to work fine, even within the same page!

WordPress database error: [You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AND wp.id = wpr.post2_id AND wp.post_status IN ('publish','private') ) AS tab1 ' at line 2]
SELECT * FROM ( SELECT position2 as position_unified, wp.*, wpr.* FROM wpgtwp_post_relationships	wpr ,wpgtwp_posts	 wp WHERE wpr.post1_id = AND wp.id = wpr.post2_id AND wp.post_status IN ('publish','private') ) AS tab1 UNION ALL SELECT * FROM (SELECT position1 as position_unified, wp.*, wpr.* FROM wpgtwp_post_relationships	wpr ,wpgtwp_posts	 wp WHERE wpr.post2_id = AND wp.id = wpr.post1_id AND wp.post_status IN ('publish','private') ) AS tab2 ORDER BY 'position_unified' ASC, 'post_date' ASC

Any idea of what might be going on?

Riccardo
https://www.gurtip.it

https://www.ads-software.com/extend/plugins/microkids-related-posts/