This issue happened to me within the last 48 hours, and it has evolved. I’m also now unable to perform live previews and then activate new themes, nor, as you describe, am I able to add new themes. All the links on the Add Theme page are dead and I can’t see any themes to browse through or add. Reinstallation provides a short fix, but after each new install, the problems described escalate progressively faster than each time before.
Looking at my install from within their internal network, my host’s “support team” sees no problem, although their customer service does. The issue manifests on Safari for OS X and iOS, Firefox for OS X and Windows, Chrome for Windows, and Puffin Pro for iOS.