You may want to check the following:
1. Are you using SSLVPN or IPSec VPN?
2. If you are using SSLVPN, is split-tunnel enabled?
3. If you do not have split-tunnel enabled, please check if you have a policy that allows VPN range IP to access to WAN
4. If you do have split-tunnel enabled, you may want to check your local routing of Ubuntu machine. When split-tunnel is enabled, the route towards the Internet should use the local Internet of your hosts.
You can also check the debug flow when you are connected to the VPN by running the following command in FortiGate CLI:
diag deb flow filter saddr <source_IP_of_VPN>
diag deb flow filter daddr 188.8.131.52
diag deb flow filter proto 1
diag deb flow sh function-name ne
diag deb flow sh iprope en
diag deb flow trace start 20
diag deb en
Once the above is in place, from the Ubuntu machine, do a ping to 184.108.40.206, you should be able to see whether the packet is being processed or dropped by FortiGate.