Hi, have you been able to get it to work? I am having the same error.

My Setup:

Beryl Ax3000 = the VPN wont connect. With the same error as yours
Using AX1800 as the vpn server.
My ISP provided me a static IP, so i did a port forwarding on the router, assigning a port to the IP of the Flint Server.

Please i need some help as i am not a network expert on this.
I also tried 'nc -zv my ip and the assigned port" but got Connection refused.