How to fix this warning

The password has been marked as 'EXPIRED' or marked with an 'EXPIRY_DATE' in dba_users. You will have to change it. You can set it back to the same password. The easy way would be setting the password "by values". This usually bypasses history checking.

12:28:33 SQL> select * from dba_users where username = 'MYUSER1';

USERNAME                          USER_ID PASSWORD
------------------------------ ---------- ------------------------------
ACCOUNT_STATUS                   LOCK_DATE          EXPIRY_DATE
-------------------------------- ------------------ ------------------
DEFAULT_TABLESPACE             TEMPORARY_TABLESPACE           CREATED
------------------------------ ------------------------------ ------------------
PROFILE                        INITIAL_RSRC_CONSUMER_GROUP
------------------------------ ------------------------------
EXTERNAL_NAME
--------------------------------------------------------------------------------
MYUSER1                               338 66856982BE5CD23F
OPEN
USERS                          TEMP                           17-JAN-11
DEFAULT                        DEFAULT_CONSUMER_GROUP



Elapsed: 00:00:00.03
12:28:43 SQL> alter user myuser1 password expire;

User altered.

Elapsed: 00:00:00.00
12:29:11 SQL> select * from dba_users where username = 'MYUSER1';

USERNAME                          USER_ID PASSWORD
------------------------------ ---------- ------------------------------
ACCOUNT_STATUS                   LOCK_DATE          EXPIRY_DATE
-------------------------------- ------------------ ------------------
DEFAULT_TABLESPACE             TEMPORARY_TABLESPACE           CREATED
------------------------------ ------------------------------ ------------------
PROFILE                        INITIAL_RSRC_CONSUMER_GROUP
------------------------------ ------------------------------
EXTERNAL_NAME
--------------------------------------------------------------------------------
MYUSER1                               338 66856982BE5CD23F
EXPIRED                                             17-JAN-11
USERS                          TEMP                           17-JAN-11
DEFAULT                        DEFAULT_CONSUMER_GROUP



Elapsed: 00:00:00.03
12:29:14 SQL>

The format of this is "alter user USER identified by values 'hash from dba_users.password';"

[TEST] C:\>sqlplus system

SQL*Plus: Release 10.2.0.4.0 - Production on Mon Jan 17 12:18:16 2011

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

Enter password:

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

new: showmode BOTH
12:18:17 SQL> prompt end of LOGIN.SQL
end of LOGIN.SQL
12:18:17 SQL> create user myuser1 identified by mypassword1;

User created.

Elapsed: 00:00:00.01
12:18:21 SQL> grant connect, resource to myuser1;

Grant succeeded.

Elapsed: 00:00:00.01
12:18:30 SQL> connect myuser1/mypassword1
Connected.
12:18:39 SQL> connect system
Enter password:
Connected.
12:18:51 SQL> alter user myuser1 password expire;

User altered.

Elapsed: 00:00:00.00
12:19:05 SQL> connect myuser1/mypassword1
ERROR:
ORA-28001: the password has expired


Changing password for myuser1
New password:
Retype new password:
Password changed
Connected.
12:19:16 SQL> connect myuser1/mypassword1
ERROR:
ORA-01017: invalid username/password; logon denied


Warning: You are no longer connected to ORACLE.
12:19:21 SQL> connect system
Enter password:
Connected.
12:19:34 SQL> alter user myuser1 identified by mypassword1;

User altered.

Elapsed: 00:00:00.01
12:19:49 SQL> alter user myuser1 identified by mypassword1 password expire;

User altered.

Elapsed: 00:00:00.01
12:20:26 SQL> select username, password from dba_users where username = 'MYUSER1';

USERNAME                       PASSWORD
------------------------------ ------------------------------
MYUSER1                        66856982BE5CD23F

Elapsed: 00:00:00.01
12:20:36 SQL> alter user myuser1 identified by values '66856982BE5CD23F' ;

User altered.

Elapsed: 00:00:00.01
12:21:01 SQL> connect myuser1/mypassword1
Connected.
12:21:13 SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

Have you checked your ORAPWD file? Maybe the initial originator of the database set entries=1. That would allow 1 user to logon as SYSDBA and the second user would not be able to logon as SYSDBA. Just a thought ;-)