When trying to connect on Windows, an error message is returned:
A connection between your computer and the VPN server has been established but the VPN connection cannot be completed. Error Code 806 – The most common cause for this failure is that at least one Internet device (for example, a firewall or router) between your computer and the VPN server is not configured to allow Generic Routing Encapsulation (GRE) protocol packets. If the problem persists, contact your network administrator or Internet Service Provider.
On Linux systems:
LCP: timeout sending Config-Requests
Install OpenWrt package: kmod-nf-nathelper-extra.