hem... that was not so difficult, finally... (sorry I just begin to learn PHP & Elgg development)
Just put in comment line 37 : "var_dump($entities);" in file "/views/default/widgets/recommendations_received/view.php"
Now it's working perfectly.
Many thx Liry, you did a great job with this plugin !!!
@christophe - oh, just spotted it! I didn't even realize from your english that this is the error that you were referring to... thanks, it's fixed now.
not sending notifications...