Tasks deleted at sprint end
Regarding this: "All unfinished Stories are then moved back to the Product Backlog and all unfinished Tasks are deleted."
I would argue that the tasks in a story shouldn't be deleted simply because the story wasn't completed within the sprint. A bit of work goes into those tasks, and it's more likely that the users will just recreate or work around this feature. You could put the tasks into a special state that requires them to be confirmed again, the same way you put stories into a special state when they are kicked that requires them to reconfirmed.
Statement from Kunagi Team
The tasks are not deleted. They just are not visible. When you pull your story into a sprint again, the tasks will be visible on the whiteboard again.
Issue is closed.