How to implement PPTP connection from an intranet through the gateway on isa 2006?

Question

Network administration

Good afternoon
In one of the remote branches we have the following:
Purpose: to install pptp from the local network, as an isa 2006 gateway. Error 806 is given to clients (clients have windows 7 enterprise everywhere).
Windows 2003, ISA 2006.
Created a system policy for the GRE protocol (ip protocol, send-recieve, 47).
Created a rule for VPN (added all client protocols PPTP, L2TP, ipsec, IKE and added the created policy for gre there). From internal to VPN server.
Error 806 is given to clients, in the logs the situation is as in the screenshot. After rebooting the gateway, everything starts working, but after a certain time, the vpnka again falls out into error 806 and the same entries in the logs, and it often happens that everything works for one or two people, for the third 806 (and until the gateway is rebooted, the error persists). Reboot of the microsoft firewall service often helps.
I did the following:
I installed the Fix391667 update, it did not help.
Enabled/disabled receive side scaling for clients (both in OS and adapter drivers).
I tried various options for creating rules (allowed all traffic to the vpn server, created a rule from a specific computer to the server).
One way or another, at a certain moment, gre failed connection attempt starts to pour into the logs and a response occurs with an 806 error. Moreover, this can happen on one computer, and a couple of others can be online (disabling them also does not help to connect).
In which direction to dig?

Answer 1

Screenshot of logs at the time of connection