VPN error 806: GRE protocol 47 blocked, and why PPTP should be retired
Error 806 means GRE protocol 47 is filtered along the path. Allow GRE as a stopgap, then retire PPTP for IKEv2 or SSTP on a supported VPN server.
Bitdefender VPN error 806: the tunnel drops or refuses to start at all
Error 806 means GRE is blocked, and Bitdefender VPN does not use GRE. Here is where the number really comes from and what actually to fix.