I'm currently facing the same issue. I did everything, clear cache, login to other regions. Still failed. I tried using Firestorm viewer, but failed too.
I can only login in several conditions below.
1) Use Speedlight
2) Use an older SL viewer (6.6.16.582093) that I had in an older PC
3) Login with an old account. But that only works for the first time. Subsequence login also failed.