I could connect to a Windows 7 machine using Remmina remote desktop client on Ubuntu 14.04. After few days or weeks suddenly it stopped working. I got this error: Unable to connect to RDP server.

In my case deleting entry from ~/.freerdp/known_hosts worked for me. I guess remote host Identification got changed for some reason.

Borrowed from: askubuntu.com/questions/157723/cannot-rdp-to-windows-7-with-remmina-on-12-04/214498