A moodle 2.6 upgrade boom or small explosion

I upgraded one of my moodle instances last week from moodle 2.4 to 2.6, I did skip the 2.5 upgrade step and that could have been the problem, but not likely.  I think I am learning to not skip instances when upgrading.  This is what happened.

The site upgrade went fine, took about 30 minutes total.  After the upgrade was complete, I logged into the site, checked the front page, accessed a course or two and that's about it.

On Monday, I have a emergency email from my customer saying a teacher and her, in the admin role, could not access a course.  About 50 courses on the site, 49 seemed to be ok, but it only takes 1 to create a big problem.

I detailed the problem here,  in the Moodle general help forum and got zero help....Which says to me, this is a bug that is not documented, I should probably submit something to the developers.  Anyway, I digress.

When clicking on the course in question, the browser would spin and then present a reset request page.  I tried in 3 different browsers and had same behavior.

I spend about 3 hrs, trying to create a backup of the course, restoring each attempt with the hope of different behavior.  Each time the course would backup fine, and restore fine, until the last step of the restore process, which is to open the course in a view, which is when it would fail with the same behavior.  Each backup, I would try reducing the content, hoping I could find something that worked and at least the content that was responsible for the failure.  Basically, each backup, I would remove a section and its content from the backup.  Nothing worked.

I then spend about 3 hrs rebuilding the course from scratch.  I was able to access the course content of the failing course by going into the Site Admin | courses | Manage courses and categories and clicking on the edit link.  This did, thankfully, allow me to see the content so I was able to copy/paste - export/import lots of files.  I had to rebuild the activities, which were

• forums
• assignments
• glossary
• wiki
• quiz
• lesson

The wiki, lesson and glossary are not commonly used activities (unlike quiz, forum and assignment which are) and make me a little suspicious, but I have no hard evidence that they were the culprit.

Fortunately, there was not much student activity in the course, because that was lost.  I had to re-enroll the students into the new course, so any progress or input in the original course is gone.

This is the browser response:

Moral of the story?

Sometimes, moodle does strange things, and people do not know why.  Use incremental upgrade steps
Do not skip a version when upgrading.