I've used IE6 since back in the beta/preview stages and have never had problems like that. Have they cleared out their cookies and stuff? If they're upgrading, it may be a problem with cookies from the old version not liking cookies from the new version or something. Have them clear their cookies from your site from the profile link. If that doesn't work, have them clear their browser cache and delete all their old cookies to start fresh. That's the only thing I can think of.
---Skorpion
