Suddenly I couldn’t connect to my Windows 7 machine by RDP using Remmina on Ubuntu 12.04. It had worked before and suddenly stopped working. The error message displayed was ‘Unable to connect to RDP server’. Other RDP connections worked fine.
For some reason the problem was solved by editing the file ~/.freerdp/known_hosts and deleting the line with the server having the problem. When connecting again I was asked to accept the certificate and now the RDP connection worked again.