The only thing i can think of is OpenVPN is possibly changing the local ip address of the server. I know on my iMac, when i connect to a VPN it changes my LAN IP from 10.0.0.2 to something like 10.9.0.62 so i am wondering if it is doing the same for the server.
if your comfortable using terminal commands, i would do the following to test
in putty ( or whatever you use to connect to the unraid server ) type the following
ifconfig
paste the output here, then start up OpenVPN and do ifconfig again and paste that output here also so we can compare the servers LAN IP
Scott