[Serusers] clients not responding to challenges?

Hi all, does anyone know what the reason might be for SIP clients (XLite and other) on certain Ip addresses not responding to the www_authenticate() challenges sent by the Sip proxy? I can see (with ngrep) that the clients send register requests to SER, and I also see SER sending back challenge packets. Anyway, no challenge response is sent back from the client, but a normal REGISTER. XLite diagnostic log does not show any challenge request packets being received by the client. My first assumption would be a routing problem between the ISPs networks, but this all happened suddenly after a restart of my SER machine, even though nothing on the configuration has been changed. On some machines(resp. ISPs), the client registers successfully and can dial out other SIP clients, but does not receive any incoming calls. On other - everything works perfectly both ways. Pls give me any clues or suggestions if u can...