Sorry to bump this message again but im afraid that if i cannot solve this 
issue I will have to replace the m0n0wall with our Cisco 1760 which i really 
do not want to do. Anyone any idea what is going wrong? If i need to provide 
additional information please let me know.

Thx

I'm using m0n0wall version 1.2b3 on a wrap board. From our provider we 
receive a subnet of 16 ip's which we can use. The m0n0wall is acting as 
router and firewall for this subnet. I am using the 3 interfaces as folowed.

WAN 81.*.*.*/30
LAN 10.*.*.*/24
DMZ 81.*.*.*/28

I have disabled NAT by enabling advanced outbound NAT for the DMZ and added 
a mapping to enable NAT for the lan interface.
One of the devices on the DMZ interface is a Cisco trying to run a GRE VPN 
tunnel but it seems as if the m0n0wall is blocking the tunnel. 
When i look into the firewall logs it doesnt display anything blocked.
The m0n0wall itself is running the PPTP server which is running just fine 
(also with GRE protocol).

Does anybody have an idea why i cannot create the tunnel with that Cisco? 

Thx.

Carlo