This may sound silly, but have you tried a different browser? Some browsers can affect what information is entered in various dialogue boxes (such as password field).
With current versions of the Symantec Messaging Gateway (9.x), the password for the default admin account is the same for both the Control Center web GUI as well as the Command Line Interface (CLI). This means that you can use the password command from the command line to change it:
http://www.symantec.com/business/support/index?page=content&id=DOC2528
However, that might not affect your situation since you can already log into the command line. It may be worth it to also try restarting the controlcenter and mysql services using the service command (CLI) before attempting to log in again.
If the issue persists, take note of any errors you are seeing and contact support for further assistance.