More or less happy to read this. I've commented on this issue somewhere else a couple of days ago, ignorant of this thread which is of course the more appropriate one to do such things.
I've been having this problem for .... weeks now already. Both on OSX and Windows machines, on every browser (Firefox, Chrome, IE, Edge, Safari).
My work around is that after the login/password and getting a "wecome back", only to see a "not logged in status", I press the "help" thingy at the right upperhand corner, and voila, I'm logged in.
Clearing cookies/resettings browsers and what have you not didn't help, or only helped one or two times.
Took me some time to figure that one out though. 
But I suspect the problem pops up when you don't leave the site "as supposed to do", ie, not using "logout". Cause once logged in, if I "correctly" use the "logout" button, a new attempt to login seems to work flawlessly every time.
