Original Post : http://cookbook.fortinet.com/ssl-vpn-troubleshooting/

Tunnel-mode connection shuts down after a few seconds

This issue can occur when there are multiple interfaces connected to the Internet (for example, SD-WAN). This can cause the session to become “dirty.” To fix this, you must allow multiple interfaces to connect without issue.

If you are using a FortiOS 6.0.1 or later, use the following CLI command:

config system interface
  edit <name>
    set preserve-session-route enable
  next
end

If you are using a FortiOS 6.0.0 or earlier, use the following CLI command:

config vpn ssl settings
   set route-source-interface enable
end