We're currently aware that there may be an issue since the development release on the 9th of August that may affect Design Challenge applications.
At present, you can apply to a design challenge, but if you want to update your application by applying again, you may encounter an error or be unable to.
The development team are aware and they're working on a hotfix to resolve the issue which should be today or early next week if it's a prevalent issue.
If you experience this you may still report it in Feedback and Support