Very bizarre, I have not come across this yet. Have you tried a different browser on the desktop? You mentioned you were using firefox, have you tried Internet Explorer? It definitely seems specific to your desktop and there have been no other reports of this. You said you checked the status of your cookies. I would go into firefox properties and purge all cookies if you can then restart firefox. This sounds like a corrupt cookie to me and the best way around that is to wipe it out, we use multiple cookies as well so you might not even see the one causing a problem, that's why i recommend telling firefox to delete all cookies and cache..

Sean