Gracias a Eloy he encontrado esta solución que funciona impeke:
I've fixed this with my own hack. Edit the files:
moodle/mod/quiz/report/overview/report.php
moodle/mod/quiz/report/analysis/report.php
and replace all occurances of simply "JOIN" with "LEFT JOIN" which is what was used in the earlier 1.5.2 version. Since I did that I've had no further problems.