I am sorry about the confusion. (overlooked with OpenVPN)
Looking about this compatibility problem I found the following information on the OpenDNS vendor site:
The IP Layer Enforcement feature of the Roaming Client is incompatible with:
Built-in OS X VPN client
> Fortinet FortiClient
SonicWALL VPN (some environments)
It is known to be compatible with the following VPN Clients only. If it is not on this list, and you are experiencing an issue, disable IP Layer Enforcement and confirm if the issue also resolves.
There have been multiple similar issues in the past reported by customers ( between OpenDNS and Forticlient). On the paid-license version you can try changing Forticlient's control of the DNS Cache Service under VPN>SSL VPN in the EMS profile. Unfortunately you have an issue on the free version where this option is not available.
Fortinet Technical Team Lead
NSE 1-4,7 Certified