Comment # 11 on bug 1194193 from Sotir Danailov

This still happens to me as well, but only on Firefox. With Chromium I seem to
be able to login every time. At first I thought that might be related to
extensions, since in Firefox I have a lot, but when I disable all of them and
try again it's the same thing. Extensions haven't interfered with other logins
so far. My guess is it's not that. The weird bit is, in Firefox sometimes
randomly it works, but rarely. It's very hard to tell what the cause is, but if
I find a clue I'll post it here.