I also get a 404 using Firefox. With IE it redirected to
this page, which prompts me for Invitation Code and Last Name.
Handing
that link to Firefox does work. But note the "AAPLATNPS" in it. I'm almost positive that IE would have no way of knowing I'm Plat, since I only use it for the very rare site (never AA.com) that isn't working right in Firefox. It's possible (but unlikely) that Citi knows.
If I strip off that final parameter (everything from the last "&" on) I get what appears to be the same page.