how to login to vpn server over a RouterOS as gateway

hi, every body,
I have a strange error, when I try to login to a VPN Server,
Client(WIN2003 or ROS with IP 192.168.1.123) ↔ ROS 2.9.6(as GW with 2eth and 2IP: 192.168.1.1, 192.168.0.2/24) ↔ D-Link Router(as ADSL Dialer, with LanIP 192.168.0.1) ↔ VPN SERVER(pptpd on Ubuntu v10).

and the error is:
Client can connect to VPN-SERVER, can ping, can telnet VPN-SERVER 1723 port, but can’t login, the log of the pptpd(on VPN Server) is:

Connect: ppp0 ↔ /dev/pts/5
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x8f32a9cc> ]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x8f32a9cc> ]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x8f32a9cc> ]

no any rcvd statement

If I remove the ROS 2.9.6, in other words, Client ↔ D-Link Router ↔ VPN SERVER(pptpd), can successfully login and every thing is OK.
If I don’t remove the ROS 2.9.6, but change the VPN-SERVER as “Route And remote access” on WIN2003, can successfully login and every thing is OK.

Is anyone know how to deal with the strange error, thanks very much!

Oh, the VPN-SERVER(pptpd on Ubuntu) is work properly, bcz I can connect and login it from Other Terminate.