We can now confirm that the ongoing issues affecting new installations and updates are caused by a sustained cyberattack on Canonical's infrastructure (the company behind Ubuntu Linux). This is completely outside of our control.
Unfortunately there is no ETA for recovery as Canonical are actively working to resolve the attack. We are working on an alternative solution to bypass their infrastructure entirely and will update this notice as soon as we have more information.
We sincerely apologise for the inconvenience and thank you for your patience.