Can't connect to RDP Bookmark with domain user

Hello,

I have and strange problem with Fortigate HTML5 Bookmark. I can connect with a local user account to the RDP, but can't connect with a domain user account. The same domain account is accessible outside the fortigate.

The server machine is a Windows 7 and Fortigate version is v6.0.5. I've configured the security as "let server choose". Any ideas?

Regards