As I have indicated above, the default in 10.4 is "cyrusimap" and not "admin"
If the name there is instead "admin" it should not be, and even then I advise against using "admin" as the actual account name for your main admin account.
If you are attempting to use cyradm, then your configration is not as it should be.
Read up at
http://www.afp548.com/article.php?story=20040814204411280
"...you should create an IMAP administrator account... There is no need for this user to be an actual admin on your server, so just leave it as a normal user."