I have a multilingual site and the related posts only display properly for my primary language (English), but not the secondary language (German).
I tried clearing the YARPP cash and it worked well, but only for one instance and after refreshing the site just one time the related posts are messed up again. On some articles there are even double posts displayed.
So everything is fine for first language, but not for the second.
I'm running WPML and HyperCache, so there may be a conflict with these plugins, but I would hate to have to disable any of them.