I have Lumia 900 as backup phone. Recently I found that I am not able to download any app from marketplace. I can open marketplace, search apps, but as soon as I hit "Install" or "Update" button, it leads me to a page to create live account, but I already have account set up.
I logged into windows phone.com on PC and brought up my arrive.
Now I was able to install the 4 updates. I also download wincentral app. And other apps.