PPTPD connect issue

Hello all,

i have a strange issue with my pptpd v1.3.4 setup on Ubuntu server 10.04, and would be extremely grateful for assistance or insights.

Users can connect to the VPN fine for days at a time, using different devices and usually behind regular office nat routers connected to the internet. Everything is great. Then all of a sudden, while 1 User may still be able to connect via wifi OR wired (not both anymore), nobody else can. Connecting from another network then (say the neighbors wifi), it will work just fine for everyone who tries.

When using tcpdump on the pptpd server side, i can see a clients incoming connection, and i can see the server sending back configuration replies or ack packets. When then using tcpdump on my LAN side WAN Interface, i can see what looks like the correct packets arriving back at the firewall, but they never make it anymore to the client, so i believe whats happening is that a connection never gets negotiated: the Client is sending configuration requests, the Server is sending back answers, but the LAN firewall will never pass them on somehow. Sounds like a gre issue? But nothing changed and things may work fine for days until this issue pops up.

While having this odd situation, i rebooted the pptpd server (which is on a fixed Internet IP somewhere in another country), rebooted my clients (such as iPhone, regular PC, MAC, Windows, all behind a Linksys NAT Router), rebooted the Linksys, all to no avail. Tried using clients on wifi and wired. It just won't connect no more.
As a last resort then, i turned off the power to the whole office server rack, which basically just contains the Linksys, a dumb 24 Port Switch, and a Modem. Resetting the Switch made everything go back to normal, and everybody could connect again.

I have had this issue now from inside 3 different networks with different setups and different clients. I don't know the exact steps yet to trigger this "unable to connect" problem, but the next time it happens i'll restart the switch again and believe it will temporarily solve it.

Is anyone available to help me troubleshoot this rather bizarre issue? I have taken packet captures from every angle and can provide any required details. For now, just my simple pptpd.conf attached as below. 

/etc/pptpd.conf
option /etc/ppp/pptpd-options
logwtmp
localip 192.168.111.1
remoteip 192.168.111.101-200

/etc/ppp/pptpd-options
name pptpvpn
refuse-pap
refuse-chap
refuse-mschap
require-mschap-v2
require-mppe-128
ms-dns 8.8.8.8
ms-dns 8.8.4.4
proxyarp
nodefaultroute
debug
lock
nobsdcomp 


best regards,
David S.

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