Although it sounds like a server error, because no one seems to be reporting a problem it suggests it's local you.

So I would suggest opening Internet Explorer and clearing the browser cache. Even if you don't use it as your main browser, the game use the built in Windows browser which is Internet Explorer. After doing that, try launching the game again.

This is assuming that after Steam reverted it back to 2.1 you let the launcher repatch all the way back to 3.15? Or is this before you have started to patch it yet?