[Serusers] ACK Message problem

Hi, I was making test call from a UA1(10.0.2.12) via SER (192.168.168.87) to another UA2 (192.168.168.33) where UA1 is a program written by us and UA2 is a softphone. When I cancel the call, I am having problem sending an Ack to SER. In the trace attached, when UA1 send a Cancel, it replies with 200 canceling in response to this Cancel, and 487 in response to the original INVITE message. Then UA1 send back an ACK in response to 487 but SER keep resending 487 message. It seems like SER doesn't recognize the ACK message for some reasons. Could anyone tell me what is the problem with this ACK message please? Below are the only errors I can see in the /var/adm/message when I start up ser. There is no error message during the test call. Nov 11 09:07:12 TakeAway ./ser[5557]: [ID 212014 daemon.warning] Warning: uac does not spread across the whole hash table Nov 11 09:07:12 TakeAway ./ser[5557]: [ID 499528 daemon.notice] Maxfwd module- initializing Nov 11 09:07:12 TakeAway ./ser[5558]: [ID 827449 daemon.warning] WARNING: no fifo_db_url given - fifo DB commands disabled! OS version: Sparc Solaris 5.9 Ser version: ser 0.9.3 Thanks, Eddie