The Asset Store simply says "error" when trying to download anything on Unity 5. ??

It works for me so I'd suggest making a thorough check of firewalls and so on. Standard answer but it is also very common error.

I would also make sure your drives have enough space and then some. I'm not sure how Unity handles downloads but it is always nice to make sure it isn't a full drive messing things up.

Having the same Problem with 24mb asset. So i dont think it is a diverse problem. 1tb of my 2 are still free to use. Maybe its just a temporary Problem because of the sale. Would Guess that when you are trying to download something for free the store tries to get 75% off that product. And 75% off 0 leads to error. Just assuming.