Additional error case:
I had a group with two users. One user had an lower case user name and the
second user name was all capital letters. I permissioned read access for the
group using Matrix Based Security. User one was able to login, but not user two
(all caps). After configuring the second user separatly, he was able to login.
This was on a Windows XP box. On a Windows Server box, everything was fine after
permissioning only the group. No need to permission the second user (all caps)