After upgrade Moodle 1.7.2 to 1.7.3 the grade of all courses are not visible, the string "nogradesreturned" are showed.
A think so about the problem is in the function grade_get_course_students in file grade/lib.php!!

Version 1.7.2:
if ($rolestoget = explode(',', $configvar)) {

Version 1.7.3:
if ($rolestoget = explode(',', $configvar->value)) {

I´m wait for the answer