Hi thank you for flagging this - our team are looking into it and I will report back when resolved.
To unblock you in the meantime you can still use the old sandbox by going to learn.kx.com in a different browser in private browser mode and entering classroom ID = academydemo. Once loaded you can adding /ax to end of the URL to navigate to the Developer instance and open the capstone classroom.
Sorry for this inconvenience and please bear with us as we sort out these teething issues with the new sandbox.