by Tim Hunt.
Some things to try to get more information:
1. Look in moodledata/upgradelogs, and there should file called qe_{datestamp}.html which contains information about what the upgrade did so far. However, it probably won't contain the error.
2. Go back to the backup of your pre-upgrade site, and try to review the attempts of the quiz with id 2. That is, go to .../mod/quiz/view.php?q=2, go to the reports, and review all the attempts. Are there any errors before the upgrade?
3. In the pre-upgrade site, got to the 'Question engine upgrade helper' tool in the admin menu, and use that to upgrade the attempts before doing the main upgrade to 2.6.