It is not possible to cancel a huge task tree (more than 1000 tasks) from Provider's Control Panel in OA. Trying to do so will lead to Apache timeout. Subsequent attempt to cancel the task tree may result in hanging of task manager due to high CPU consumption of SaaS SoLoader on the Management Node.
This behaviour is caused by a limitation known as a software issue POA-79243: Cancel tasks tree does not work when canceling a task with too many dependent tasks.
Do not try to cancel tasks with many dependent tasks. If such action is necessary, please contact Odin Technical Support.