The Mendeley API seems to have changed the way it handles authors, but this the code in the plugin. They have changed from returning authors as a string to returning them as a JSON array of 'forename' and 'surname'. This is not yet in the API documentation, but the bug on it has been closed: https://mendeleyapi.16bugs.com/projects/6677/bugs/193954
Version: Mendeley Plugin 0.6.5 on WordPress 3.2.1
I haven't done thorough testing, but fixed my installation by changing:
$grpval=$grpval->surname . $grpval->forename;
$authors .= $author_arr[$i]->forename . " " . $author_arr[$i]->surname;
I haven't tested yet to see if the editor listing is also changed (for line 401).