Hello,
Unfortunately we can cannot connect to 6.5 host from C# Client as this is now decommissioned. Even if you try to connect to 6.5 Host from Thick client it will just fail.
I did not find any other way to fix this.
Thanks
Sorry, you no longer have that option with the C# client. Ensure you keep your ESXi embedded host client current with the latest version on the Flings site and you'll have a much better experience.