Inconsistent display of related posts

While I don’t think it’ll solve the issues that you’re having, you can fix that error message by changing YARRP_Cache.php to this on line 330:

if (defined('WPLANG')) {
			switch ( substr(WPLANG, 0, 2) ) {
				case 'de':
					$lang = 'de_DE';
                    break;
				case 'it':
					$lang = 'it_IT';
                    break;
				case 'pl':
					$lang = 'pl_PL';
                    break;
				case 'bg':
					$lang = 'bg_BG';
                    break;
				case 'fr':
					$lang = 'fr_FR';
                    break;
				case 'cs':
					$lang = 'cs_CZ';
                    break;
				case 'nl':
					$lang = 'nl_NL';
                    break;
				default:
					$lang = 'en_US';
                    break;
			}
		}else{
			$lang = 'en_US';
		}

I got the same error than @zactoff reported:

Notice: Undefined variable: lang in pathtosite\wp-content\plugins\yet-another-related-posts-plugin\classes\YARPP_Cache.php on line 359

I easily fixed it not by editing the plugin’s file, but in addind a missing line to my wp-config.php file (my projet is in French, thus the fr_FR setting):

define('WPLANG', 'fr_FR');

cf. the Codex about this.

NB: editing a plugin’s core files is a bad practice, because your changes would be overwritten with the next update.

———–

Edit: it seems that WPLANG is in fact deprecated since WP4 (cf). So the modification to wp-config.php is useless, despite the fact it fixes the PHP error caused by YARPP.

Still looking for a solution to fix this.