Moving to Esxi 6.5 is having firewall/networking issues

I moved from Esxi4 to 6.5 and everything works fine except the VPN connection. To establish the VPN connection I used Endian Firewall 2.3 that is installed as a virtual machine inside Esxi.

Before this upgrade my IT infrastructure used ESXi 4 and everything worked; after the upgrade to ESXi 6.5 the openVPN (provided by Endian Firewall) can establish the connection but the PCs inside the Intranet are not reachable (neither PING nor RDP works) by the client from outside. I tried using the same VM (phisically) of Endian Firewall but also installing the last version: the problem is the same.

Does anyone has an idea of what could be the reason of this problem? What is changed in the security/networking model of ESXi 6.5 (compared with 4) that could cause this problem?

Thanks in advance

Roberto