Environment: ubuntu / apache / postgres 9.3

Install Moodle : MOODLE_27_STABLE branch
As admin create a course + a quiz
Install grade_me block : MOODLE_27_STABLE branch
Add block to the course

You see this error:

Debug info: ERROR: column "mdl_question_attempt_steps.userid" must appear in the GROUP BY clause or be used in an aggregate function
LINE 7: SELECT userid, questionattemptid, MAX(sequencenu...
^
SELECT bgm.courseid, bgm.coursename, bgm.itemmodule, bgm.iteminstance, bgm.itemname, bgm.coursemoduleid, bgm.itemsortorder, qa.id state_id, qa.userid, qa.timemodified timesubmitted, q.id submissionid, qas.sequencenumber
FROM mdl_quiz_attempts qa
JOIN mdl_quiz q ON q.id = qa.quiz
LEFT JOIN mdl_block_grade_me bgm ON bgm.courseid = q.course AND bgm.iteminstance = q.id
JOIN mdl_question_attempt_steps qas ON qas.questionattemptid = qa.id
JOIN (
SELECT userid, questionattemptid, MAX(sequencenumber) as maxseqnum
FROM mdl_question_attempt_steps
GROUP BY questionattemptid
) maxsubq ON maxsubq.questionattemptid = qa.id
WHERE qa.userid = $1
AND qa.state = 'finished'
AND maxsubq.maxseqnum = qas.sequencenumber AND qa.timefinish != 0 AND maxsubq.userid = qas.userid
AND qas.state NOT IN('mangrright',
'gradedright',
'gradedwrong',
'mangrwrong') AND bgm.courseid = $2
AND bgm.itemmodule = 'quiz'
[array (
0 => 5,
1 => 2,
)]
Error code: dmlreadexception
Stack trace:
line 443 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 244 of /lib/dml/pgsql_native_moodle_database.php: call to moodle_database->query_end()
line 723 of /lib/dml/pgsql_native_moodle_database.php: call to pgsql_native_moodle_database->query_end()
line 109 of /blocks/grade_me/block_grade_me.php: call to pgsql_native_moodle_database->get_recordset_sql()
line 294 of /blocks/moodleblock.class.php: call to block_grade_me->get_content()
line 236 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()
line 993 of /lib/blocklib.php: call to block_base->get_content_for_output()
line 1045 of /lib/blocklib.php: call to block_manager->create_block_contents()
line 361 of /lib/outputrenderers.php: call to block_manager->ensure_content_created()
line 45 of /theme/clean/layout/columns3.php: call to core_renderer->standard_head_html()
line 877 of /lib/outputrenderers.php: call to include()
line 807 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line 243 of /course/view.php: call to core_renderer->header()

This may be a fix: http://stackoverflow.com/a/19602031 ( I didn't look at the big SQL query so it may not be a valid fix at all)

PS: thanks for the plugin, it's a great feature!