Along with the AFS password expiration of 120 days put into effect on 09 April 2007, there is a limit of 5 consecutive failed logins within an approximately 30 minute period, which will will lock out a user for about 30 minutes from the time of the fifth failed login.
Typical causes of repeated failed login attempts are email (IMAP or POP) and/or ftp clients on your computer, that are using an incorrect AFS password for your account.
Remember to change all client application passwords that authenticate to AFS to your current AFS password every time you change your AFS password.