Poptop question

I installed Poptop on my redhat.
I use Win2K built-in VPN client to connect to redhat.
However, sometimes the vpn connection can establish, sometimes cannot.
It shows the following message when vpn connection can't be establised

Dec 29 11:07:06 localhost pptpd[29267]: GRE:
read(fd=6,buffer=8055600,len=8260) from network failed: status = -1
error = Protocol not available
Dec 29 11:07:06 localhost pptpd[29267]: CTRL: GRE-tunnel has collapsed
(GRE read or PTY write failed (gre,pty)=(6,5))