I very much envy all you people who can get rid of the error by logging out and back in - I have no doubt it works for you, but for me, on certain computers and browsers, no amount or combination of restarting the browser, logging in or out, or clearing cookies gets rid of that and other messages and allows me to use any part of the site requiring login, including mileage summary, itineraries, upgrades, or the 1K page. But on the machine I'm currently on and my laptop, everything works.

Works: On Win XP, Firefox 1.0.7 and IE 6
Doesn't work: on Win 2000, Firefox 0.9.2 (yes, I know I need to update that) or IE 6, also through a proxy (have no idea if that affects things)

Endlessly repeating the advice to logout and login while confidently asserting that it will solve all problems for the afflicted user, while seemingly helpful, and seemingly effective for some people, is only yet another frustration for some of us. I don't know what the real problem is on my Win 2000 machine, but it seems that for me, using current browsers on XP is the only way to go. I have no access to Macs to test what does or doesn't work there.