Odd VPN issue

[#1]

Hi
We have a another site connecting into our VPN however we keep getting 806 GRE issues
It seems to work fine on other isps no problem - they can dial in with no gre issues
we have IP such 10.81.48.*
They have ip 10.86.* and then go through a connection with lightspeed firewall
We are using Windows Server 2008 r2 and PPTP with PlusNet business and Technicolor TG582n

[#2]

Any suggestions?

[#3]

Hi mrcbo,
I see that you've raised a ticket on your account on this matter. I've asked that our business team takes a look at it for you and I'm sure we can offer a solution.
Tony

[#4]

When you say lightspeed firewall, do you mean Lightspeed systems? http://www.lightspeedsystems.com/en-uk/
Is it one of the Rockets or the older TTC?
If it's the rocket it might be worth temporally white-listing the IP of the VPN concentrator to see if it makes a difference, if it is the rocket we're talking about last I saw of them they were more of a web filter than a firewall.
That said I did have one block traffic from a remote managed Wi-Fi AP as p2p traffic before, took me quite a while to figure out what it was as well because with the rocket being a Bridging device I had totally forgotten about it.