Thanks for reply!!
Actually what you told is something new for me i.e remove external plugins before up-gradation.
But if we closely look the last line of cron, it is related to quiz and we dont have any external plugin for quiz based activities. But we utilized quiz activities in lot of courses.
Is there any possibility that it is happening in corruption of quiz related data in database ?? And if yes how we can rectify it or any further information which i can provide so that it can help you to assist in resolving this issue.