Do you have a problem with FortiClient SSL VPN disconnecting you randomly after a few minutes?

This issue might occur when you have multiple interfaces connected to the Internet (i.e. ECMP or SD-WAN). This can cause the session to become dirty (basically, the firewall has to reevaluate the session when routing change occurs). To prevent it from happening, you must allow the session to finish regardless of routing table changes.
If you are using a FortiOS 6.0.1 or later, use the following CLI command:

config system interface
  edit <interface_name>
    set preserve-session-route en
end

However, if you run older FortiOS, try below commands:

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

I hope that quick trick helps!