This seemed to work on Windows 8.1 (latest updates): disabling “Automatically detect settings” in the Network > Proxy control panel.
Not sure if this was just coincidence, or something else was fixed. Perhaps the new launcher is more sensitive to network issues / crappy proxy detection? I had previously tried basically everything everybody else had in this thread (repair, run as admin, disable firewall/av, uninstall/attempt to reinstall).
It’s a simple setting to test out, so it might be worth a shot.
YMMV.
Just wanted to confirm that this worked for me as well, when nothing else did. Thanks for getting to the bottom of the issue! I’m on Win 7 64-bit, FWIW.