PPTP-VPN error "LCP timeout config request"

Hello,

when i try to connect with my iphone 3G to my WatchGuard X1250-e firewall by a PPTP-VPN tunnel, i get the error "LCP: timeout config request" in most cases. Sometimes the connection is successful, but after 4 or 5 minutes the connection will break down and both sides tell me, that the connection was closed by the other side. When the PPTP-connection is successful, everything works fine. But how can i avoid the LCP timeout?

Thanks for help