Remmina had been connecting to my RDP service for several months, until it just stopped. It will still connect by IP address. Ping confirms that the domain name resolves to the proper IP, and nothing has changed on the Windows 7 or 10 machines that use the same pathway. No updates have been recently installed that would account for the change in behavior. I agree that, since my users are up and function I could just ignore this, but it does make me curious as to why, and how to revert to the original functionality.
BTW – I'm new to Ubuntu, but can generally follow detailed instructions fairly well.