inconsistent handling of domain admin logon

Grundy

I am running TeamSite 5.5.2, SP2b on Windows 2000, and have found that TeamSite behaves inconsistently if I try to login as the domain administrator. I have added the username to my master.uid file, and iwckrole always tells me that it can indeed login in the master role. But the TeamSite login screen only lets me in some of the time; other times it reports "TeamSite did not recognize the username and password you entered. Please try again." I have verified this repeatedly, so am sure that I'm not just mistyping the password. Has anyone seen this before? Any ideas what might cause it?