Hi all - same problem here too.
I am using Firestorm/Linux - Ubuntu.
I have 4 accounts. I have experienced this on and off for at least a month now.
I tried to log in with one of my accounts - initially it didn't work. I chose a different login region and it worked - but I don't think it is related to login region.
I have had this prob for at least a month and after a while I always managed to login regardless of which account or which region I logged into.
I don't think it is happening because another viewer instance is running in the background.