If the path of server 2 traffic is hitting the WAN port(s) of the Fortigate then you likely need to set up a VIP (port forward); if both LANs on each side of the fgt are connecting via an internal port, you may need to define a route to 192.168.255.254/32 directly. However, I don't think this is actually needed. I suggest checking the return firewall policy (from server2 to server1) - you will need two firewall rules for both directions of that fgt connection. Perhaps post a screenshot (san identifiable IP info) here.
NSE4/FMG-VM64/FortiAnalyzer-VM/5.4/6.0 (FWF40C/FW92D/FGT200D/FGT101E)/ FAP220B/221C