For us this old post is still an issue.
Running FortiClient 188.8.131.520 and our users can create a VPN connection. Which seems to stay stable.
VPN stays connected, Networkshares stay connected and accessible, Outlook is working.
But after several minutes Outlook is disconnected. All other connections are still working and accessible (networkshares for example). Closing Outlook and reopening does not solve it. Let Outlook reconnect does not solve the issue.
Our FortiGate is running firmware 6.4.6. (Firmware 6.4.7 is available not yet installed).
Is there a solution for this issue?
I have some kind of a solution, that is connect to the client by teamviewer and give the client a ipconfig /flushdns
Close Outlook, reopen Outlook and it works again.