A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
I had that issue a few times myself and no idea why. Once I managed to download it, anything after it was fine. Try pausing it and resume. If that...
Edit - Friday 20th November 2015 - I finally fixed the issue. For anyone else that may be having this problem, go back a video until you find what...