[Users] login problem

[unplug]

I am using mysql.so for the user login.  User can login without
problem.  After a few hours, user issues still issue a register
request as below but can't login to the system (there is no record in
the db).  Why user failed to login suddently?  I am sure the username
and password are correct.  From the log, I can see the register
request as below.  However, user login failed.

Dec 14 17:19:32 os01 /usr/local/sbin/openser[14889]: SIP Request:
method [REGISTER] from [210.202.7.17] [1833] to [203.193.146.6] [5060]