Two users can't connect two VPN simultaneously

[cit1771]

Hi, I have encountered a certain problem that I'm unable to resolve

two users located in one apartment are not able to connect to VPN at the same time, the strange thing is that they're sometimes able to and sometimes not, as far as I'm aware it looks very random.

Steps I have tried so far:

1) Uninstall and re-install VPN application

2) Downgrade and upgrade VPN Application

3) Re-install the target OS(W10)

4) Use a different network(Mobile hotspot)

The most strange thing is that it may suddenly start working few hours later and without any issues for several weeks! 

so far I have deducted that disconnecting one user and shutting their device down allows other one to connect to VPN, but the first one starts experiencing the very same problem. 

The error simply states: "VPN Connection Failure "

And there is nothing in the logs that I can find.

[marchand]

You do not provide enough details for debug . Please post the Fortigate configuration for this tunnel .

Check if you have enable NAT Traversal in your client configuration .