I have had some sporadic issues in my time with EE where a user just cannot login to the front end, it seems to be a case of initially it validates the login details, routes you to the intended page, and then by the time the user gets there, they are logged out and dealt with accordingly. I have had it myself, so i know its effected browsers that are fine of config, and passwords etc all correct.
I am now however experiencing the issue with a very important user, everything works for everyone else in every environment, and works in this case for the user in everyone but one browser (but the browser is not the problem)
Am sorry for the broadness of the question, it is finger in the air because that is exactly how i find it when it comes around. Am just hoping someone out there has experienced the same, and has an understanding of it that i have not managed to nail it to.
Have cleared caches both our end and theirs - in the past this has been the place to start.