if the VPN user can no longer reach the server with NAT disabled, that indicates that the SSLVPN client IPs are not routed in your network for some reason.
Does the server/its gateway have a route for the SSLVPN client range pointing back to the FortiGate, or at least a default route back to the FortiGate?
You could run a traceroute (CMD: tracert <destination IP>) from your SSLVPN client (with NAT disabled in the VPN policy) to see at what point the replies go missing.
That would tell you where the route back to the VPN client no longer exists and you need to provide it in some way.
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++