Oracle 11 databases users by default use a profile that is set to force password changes. I assume you're not change the password, you're just unlocking the account. THUS.... this is why it is locking every day.
as sys with sysdba do the following.
ALTER PROFILE DEFAULT LIMIT PASSWORD_LIFE_TIME UNLIMITED;
alter user "username" account unlock;
It still have the same issue. I have noticed few more things. The account in sqlplus shows "OPEN". And I get this error everyday sharply after 6 PM. Weird one, but it is. I tried at 5.55 PM, it worked. Tried at 6.05 PM, I am getting this error. I checked it for 2 days continuously.
My testing OVM Manager environment which is running in a Virtual Machine in the same configuration is not having any of these issues.