This has happened to me occasionally, usually it's due to persistant cookies, from what I can see. Did you try completely clearing your cookies from the browser?
It has nothing to do with Mac/PC/Ubuntu, FWIW - If you're trying to access via IE that's probably the problem; try in Firefox or Chrome and let us know if you have the same problem.
I made you a rant about macs vs PCs but then I eated it.