#17 Tunnel gets stuck

[kompot]

I am trying to setup poptop on a RedHat 9 box with a
recompiled kernel with the mppe_mppc module added via a
patch. (2.4.26).
I installed the most recent ppp and pptpd available
(ppp-2.4.3-0.cvs_20040527.4 and pptpd-1.2.1-1).

The redhat box is behind a router/firewall. I am trying
to connect from another network that is behind another
router/firewall. I cann connect and browse the network
just fine. But eventually the connection just freezes.
This happens for both linux and WinXP client.

Any ideas where to look? Nothing suspicious in the log
files (probably I do not know where to look).
SSH to the same box works just fine with mozilla and
other X stuff running happily through it...