I have recently created a Moodle site and did a clean install using Moodle 2.1. Students can take quizzes but not submit them, a database transaction error occurs and with debugging mode on the following is generated;
Debug info: This code can not be excecuted in transaction
Stack trace:

line 2034 of /lib/dml/moodle_database.php: dml_transaction_exception thrown
line 59 of /lib/messagelib.php: call to moodle_database->transactions_forbidden()
line 1134 of /mod/quiz/locallib.php: call to message_send()
line 1256 of /mod/quiz/locallib.php: call to quiz_send_confirmation()
line 1003 of /mod/quiz/attemptlib.php: call to quiz_send_notification_emails()
line 1096 of /mod/quiz/attemptlib.php: call to quiz_attempt->quiz_send_notification_emails()
line 113 of /mod/quiz/processattempt.php: call to quiz_attempt->finish_attempt()

I have read through MDL-28166 but could not gleen what solution was offered, if any. My Moodle site was created with 2.1 and was not an upgrade from version 1.9. I may just need this explained to me. Looked at the overrides recommendation but may not have fully understood this as I can not submit a quiz logged in as administrator, same error as listed above.