I have exactly the same problem with the same version. My forticlient console and my forticlient connection window stay black colored. But the communication against our VPN server could be established. When I press 'return' I receive the message 'Empty username is not allowed'. In fact, I can't login but communicate. It is a pure local problem.
We actually found a solution yet! We have reset and upgraded InternetExplorer to the latest Version, because Fortinet is using the same rendering engine as IE. Obviously, there was something crappy in IE and the upgrade fixed that problem permanently.
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