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