It's working fine for me now and for ~15,000 people that are online. It's not an official info, but what I've heard in the forums is that one of the API's used by the clients to check for updates (or something like that) suffered a DDoS (Distributed Denial of Service) attack and was unresponsive most of the day. That's why the client couldn't open correctly. The secondary launcher, probably, starts the client without checking this API, that's why it was working.
So, if this problem happens again, the permanent solution is This info makes sense. I always play after server save and in the last year, I had this issue a handful of times, usually on Tuesdays when they had a prolonged downtime to apply patches and updates. So I believe that this issue is related with their server being unresponsive and not a client issue, which means that there is no backup is taken).need to reinstall the client. You just to need to wait (a for them to fix the problem there. It can take a few or many hours) for the problem to be fixed by itself. No need to install a new client (and risking losing your saved minimap and hotkeys, if (or many) hours though.