error in connect to database

good afternoon,
I have a DB2 8.1 fixpack 11, S.O. AIX.
If I connect to the database db2 connect to xxxx with no fails, but if I do it this way gives me error.

db2 connect to user username using password xxxx

SQL30082N Attempt to Establish connection failed with security reason "24"
("USERNAME AND / OR PASSWORD INVALID"). SQLSTATE = 08001

if I run su - username and password does not password error.

Can anyone tell me the reason for the error?

Thank you very much and greetings.