Error: adding a new Quiz

I have Moodle 2.8.1 running on Apache and Window Server 2012.

One of my teacher couldn't add a quiz. But rest of the courses are okay. Whenever I checked the Apache error log I see this:

Potential coding error - active database transaction detected during request shutdown:\n* line 105 of \\course\\modlib.php: call to moodle_database->start_delegated_transaction()\n* line 266 of \\course\\modedit.php: call to add_moduleinfo()\n, referer: http://<mywebsite>/course/modedit.php?add=quiz&type=&course=2&section=4&return=0&sr=0

How can I fix this it?

Thanks.