Now that I think of it, your description sounds like a recurrence of that bug we have had, in which the login doesn't complete successfully. You click to log in, enter your credentials, click to submit, and when the login dialog closes, you're back at the page but not logged in. I don't remember exactly, but I think that was never solved by Kalmbach, but it seemed to go away as newer versions of the browsers were pushed out.
Clearing the browser cache was a workaround for some (like me), but not for everyone. And you tried that, to no effect.