[Serusers] problem with plain text messages

Hello, I'm running an SER checked out from cvs with almost unchanged standard config file based on [ser]/etc/ser.cfg (see attachment). Sending a short plain text message from a MS Messenger on a Win XP machine in the LAN to a kphone on the linux host running also the SER works fine. And that even though the SDP part of the request claims about an "invalid line" (btw.: you can watch the attached *.ngrep files with ethereal). Sending a message the other way around (from kphone to messenger) the messenger replies with 400 Bad Request. I made sure that the messenger's privacy settings allow any user to send messages to me. The same (reply: 400 Bad request) happens if I send a plain text message from one MS Messenger to another on two different hosts in any direction. Can you help my find out why I can't receive text messages with MS Messenger ? Thank you very much, Alex