Major Method setup_groups() in class grade_report has been changed to private in Moodle 2.0 development head when it should be public (or the grader plug-in needs fixing).
report/grader and some other grade book plug-ins extend the class grade_report and call setup_groups() witch used to be public. This means that grader and other plug-ins will throw a Fatal error: "Fatal error: Call to private method grade_report::setup_groups() from context 'grade_report_grader' in /var/www/localhost/htdocs/moodle/grade/report/grader/lib.php on line 147" when trying to show or deal with groups.
This can be fixed by making setup_groups public or protected. I am rather sure it should be one of those as no method in grade_report calls setup_groups so making it private means nothing is or can use it.