[Serusers] [Windows Messenger SIP 5.1] Problem with "Send an instant message"

Hi, My configuration : - Windows Messenger 5.1 SIP installed on 2 PCs (Windows XP) - Ser v0.9.6 installed on Mandrake 10.1 The problem is that I have this error : "The following message could not be delivered to all recipients : my message" when I send an instant message, but the voice call works very well... with Ethereal I see that the instant message receiver send these requests to Ser : - Trying - OK - BYE why does the receiver send BYE whereas the transaction is not finished ? my traces from ethereal : from Ser(192.168.10.137) to Receiver(192.168.10.127) : INVITE sip:192.168.10.127:10835 SIP/2.0 from Receiver(192.168.10.127) to Ser(192.168.10.137) : SIP/2.0 100 Trying SIP/2.0 200 OK BYE sip:192.168.10.137;ftag=6a036c30432a4e0dab11dda94916e9ea;lr=on SIP/2.0 from Ser(192.168.10.137) to Receiver(192.168.10.127) : ACK sip:192.168.10.127:10835 SIP/2.0 MESSAGE sip:192.168.10.127:10835 SIP/2.0 SIP/2.0 200 OK from Receiver(192.168.10.127) to Ser(192.168.10.137) : SIP/2.0 481 Call Leg/Transaction Does Not Exist thanks in advance, Best regards, Malik