There seem to be some server problems with Curse.com.
This will affect Curse.com and it also seems Curse Client downloads.
CurseForge/WowAce do not seem to be affected, so as a developer you can continue to update addons.
For end users, if you wish to download addons you can access them from the project page (http://www.wowace.com/addons/arl/ for example). Click on the files tab and you can download from there.
The people who make things work are aware of the problem, and are working on it.
Sorry for any inconvenience.
Update: The network issue has been resolved, and all services should be working properly again. We apologize for the inconvenience.
A network outage at a core router in New York City is currently preventing many Curse network sites and services from functioning properly, for people in certain parts of the world. We have contacted our data center, and they are looking into the issue now.