PAM Authentication failure

In my log file:

Security Violations
=-=-=-=-=-=-=-=-=-=
Apr 9 08:51:10 server proftpd[32029]: server.abc.com (1.1.1.1[1.1.1.1]) - PAM(user_A): Authentication failure.

Apr 9 08:51:10 server proftpd[32029]: server.abc.com (1.1.1.1[1.1.1.1]) - USER user_A: Login successful.
* 1.1.1.1 : Real IP
** My server : server.abc.com

I wonder why my log file stated PAM Authentication failure while the user actually has the correct password and can login successfully, what's the problem behind?

Thanks in advance.

 

 

 

 

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