The quiz review (and reports) are returning the following error (moodle in debug mode):
Error reading from database

More information about this error

Debug info: ORA-00932: inconsistent datatypes: expected NUMBER got CHAR
SELECT u.id || '#' || COALESCE(qa.attempt, '0') AS uniqueid, qa.uniqueid AS attemptuniqueid, qa.id AS attempt, u.id AS userid, u.idnumber, u.firstname, u.lastname, u.picture, u.imagealt, u.email, qa.sumgrades, qa.timefinish, qa.timestart, qa.timefinish - qa.timestart AS duration , COALESCE((SELECT MAX(qqr.regraded) FROM m_quiz_question_regrade qqr WHERE qqr.attemptid = qa.uniqueid),-1) AS regraded FROM m_user u LEFT JOIN m_quiz_attempts qa ON qa.userid = u.id AND qa.quiz = :quizid WHERE qa.id IS NOT NULL AND qa.preview = 0 ORDER BY uniqueid ASC
[array (
'quizid' => '155',
'oracle_num_rows' => 100,
)]
Stack trace:
line 391 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 268 of /lib/dml/oci_native_moodle_database.php: call to moodle_database->query_end()
line 1042 of /lib/dml/oci_native_moodle_database.php: call to oci_native_moodle_database->query_end()
line 1354 of /lib/tablelib.php: call to oci_native_moodle_database->get_records_sql()
line 321 of /mod/quiz/report/overview/overview_table.php: call to table_sql->query_db()
line 1375 of /lib/tablelib.php: call to quiz_report_overview_table->query_db()
line 391 of /mod/quiz/report/overview/report.php: call to table_sql->out()
line 90 of /mod/quiz/report.php: call to quiz_overview_report->display()

I have fixed the error (for Oracle is OK, but it should be tested for the other databases too) updating the following files:

/var/www/moodle20/mod/quiz/report/responses/report.php

188c188
< $fields = $DB->sql_concat('u.id', '\'#\'', 'COALESCE(qa.attempt, \'0\')').' AS concattedid, ';
—
> $fields = $DB->sql_concat('u.id', '\'#\'', 'COALESCE(qa.attempt, 0)').' AS concattedid, ';

/var/www/moodle20/mod/quiz/report/overview/report.php

< $fields = $DB->sql_concat('u.id', '\'#\'', 'COALESCE(qa.attempt, \'0\')').' AS uniqueid, ';
—
> $fields = $DB->sql_concat('u.id', '\'#\'', 'COALESCE(qa.attempt, 0)').' AS uniqueid, ';