Hi Filippo!
YARPP plugin (it is the background of WP Related Post plugin) uses full text search.
According to YARPP support forum, the problem can be solved if you use MySQL database server and MyISAM table type for the post table. If you use InnoDB table type, “you can change your wp_posts table to MyISAM and you’ll be all set.”
http://wordpress.org/support/topic/the-used-table-type-doesnt-support-fulltext-indexes
Here is a post, how can you do this:
http://www.plugish.com/2014/03/wordpress-doesnt-support-fulltext-indexes/
“This solution is made specifically for the InnoDB engine, if you’re getting this issue and you are trying to install a plugin, then the plugin may be built for the MyISAM engine, which means you will want to switch it. to do this, just do an ALTER TABLE query. ALTER TABLE wp_posts ENGINE = MYISAM; Then re-activate that plugin/theme and you’re good to go.”
We hope this will solve your problem.
You may see YARPP forums about further information.
Best Regards,
Peter Rath
WebshopLogic