Check the firewall settings on the NAS. It will need to be set to allow access from 10.8.0.0/24 (the VPN server's default network).
You could also try turning off all VPN clients on the router to determine whether that is the cause of the problem.
Is the NAS the only device that you cannot reach via the VPN? What about other devices on the LAN?
Are you running policy rules strict? If so try the non strict version - strict likely doesn't include the relevant route for your vpn server in the VPN client table
Welcome To SNBForums
SNBForums is a community for anyone who wants to learn about or discuss the latest in wireless routers, network storage and the ins and outs of building and maintaining a small network.
If you'd like to post a question, simply register and have at it!
While you're at it, please check out SmallNetBuilder for product reviews and our famous Router Charts, Ranker and plenty more!