Why won't Remmina connect to Windows 7 Remote Desktop?
Make sure port 3389 is opened on your firewall and router.
In Remmina edit your connection, click the "Advanced" tab and choose RDP under the "Security" option and save.
Just delete the server causing problem from the file ~/.freerdp/known_hosts
and it will work.
For me deleting the offending host in ~/.freerdp/known_hosts and changing the Security setting to NLA worked.
RDP used to work, but does not anymore for the Windows 7 machines I work with.