When into a Course Quizzes you request form Results and "Feedback" is included then following message is displayed:

"Nothing to display"

With debug options enabled you can obtains ...:

ORA-00905: falta una palabra clave

SELECT u.id||'#'|| NVL(qa.attempt, 0) AS uniqueid, qa.uniqueid as attemptuniqueid, qa.id AS attempt, u.id AS userid, u.firstname, u.lastname, u.picture, qa.sumgrades, qa.timefinish, qa.timestart, qa.timefinish - qa.timestart AS duration , qf.feedbacktext FROM m_user u JOIN m_role_assignments ra ON ra.userid = u.id LEFT JOIN m_quiz_attempts qa ON u.id = qa.userid AND qa.quiz = 12899 LEFT JOIN m_quiz_feedback AS qf ON qf.quizid = 12899 AND qf.mingrade display()

Debug deeper we found that keyword "AS" exceeds in the last clause "LEFT JOIN m_quiz_feedback AS qf". Editing "mod/quiz/report/overview/report.php" at line 410:

$from .= " LEFT JOIN {$CFG->prefix}quiz_feedback AS qf ON " .

replacing by:

$from .= " LEFT JOIN {$CFG->prefix}quiz_feedback qf ON " .

Now RUN without Error and Fine...